I first came to Washington in the fall of 1969 to spend a year working on Capitol Hill. My introduction to the city and its politics was a jolting one. A few days in, while sharing a house near Dupont Circle with friends, I took the dog of one of my roommates for a walk. As we neared the Circle, the dog tugged at his leash, yelping. I tugged back—until I saw what had made him panic. His olfactory senses were far more acute than mine, and a tear gas canister was heading in our direction—followed by others, then a large crowd of people moving toward us, followed shortly after by police in full riot gear.

A demonstration in front of the South Vietnamese embassy, on the other side of the circle, had gone awry. The dog and I quickly repaired, coughing and stumbling, to our townhouse, where we put wet towels in the doorframes and watched the mayhem ensue. “Welcome to Washington,” said one of my housemates.

Richard Nixon was President, facing a Democratic Congress, and Vietnam was the dividing issue of our time. To be sure, it (and other issues) had a sharp partisan edge. During my year on the Hill, Democratic Senator George McGovern, a leader of the anti-war forces, went on the floor of the Senate and declared, to a shocked audience in the gallery, “This Chamber reeks of blood. Every Senator here is partly responsible for that human wreckage at Walter Reed and Bethesda Naval and all across our land—young men without legs, or arms, or genitals, or faces, or hopes.” Not long after, Bob Dole, a freshman Republican, took to the Senate floor and viciously ripped into McGovern.

Yet what was striking to me at the time was that the issue of Vietnam was not actually defined by a partisan divide. To the contrary, the core support for Nixon’s Vietnam policy came not just from minority Republicans but also from powerful Democrats—Southern conservatives who occupied the key positions, chairing committees like Armed Services and Appropriations in both houses of Congress. And the strongest opposition to our Vietnam policy was a coalition that included liberal Democrats like McGovern joined by liberal and moderate Republicans like Mark Hatfield of Oregon, Jacob Javits of New York, and Javits’s New York Republican colleague, Charles Goodell.

On some domestic issues, from revenue sharing to health care to welfare, Nixon was able to cobble together a coalition that included many moderate and even liberal Democrats to replace conservative Republicans uneasy about positions that were in many cases shaped and drafted by his domestic policy advisor, Democrat Daniel Patrick Moynihan.

What characterized the parties back in that era was their breadth of ideological diversity. Democrats kept their majorities, which dated back to 1955, through a broad coalition of those Southern rural conservatives known as “Boll Weevils,” for the insect that infects cotton plants, and Northern urban liberals. Republicans had a sizable collection of moderate and liberal lawmakers, from New England and the mid-Atlantic, and the West Coast, dubbed “Gypsy Moths,” for the insect that infects hardwood trees in the Northeast. While magazines and journals measured ideology through votes, they also measured adherence to what was called the “conservative coalition,” which included a majority of Southern Democrats and a majority of Republicans. For much of the era from the 1950s up through the first of the Reagan years, the conservative coalition had sizable sway over policy outcomes. In 1982, Barber Conable, a conservative Republican tax expert on Ways and Means from upstate New York, complained to me that Reagan and his White House paid more attention to conservative Democrat Kent Hance of Texas than it did to him!

Yet by 1969-70, one could see change in the offing. A once solid South for Democrats had begun to crack during Barry Goldwater’s presidential run in 1964, and a combination of migration patterns from the Snowbelt to the Sunbelt was adding Republicans to the South and draining Republicans from the Northeast and Midwest. A regional realignment presaged a political realignment, as parties sorted and became more homogeneous and ideologically distinct.

Much has been written about that sorting, and much of the writing starts with Goldwater and the Voting Rights Act of 1965. A good deal of the analysis focuses on the dramatic regional realignment of parties, as the South moved from deep blue over time to deep red, while the Northeast and Pacific West moved from red to blue. But as Sam Rosenfeld documents in his important new book The Polarizers, the polarization that resulted did not just happen, was not simply caused by people moving from one part of the country to another—it was presaged decades earlier, and pushed by a bevy of influential people. Indeed, the impetus to move the parties from ideologically diverse coalitions toward more coherent, ideologically constrained and parliamentary-like entities began long before.

The Polarizers is a work of both intellectual and political history. As such, it gives almost equal weight to political scientists and scholars and their political counterparts who, from the 1950s on, were providing an intellectual and political framework to achieve their desired political aim: a liberal Democratic Party, freed from the obstructionism of Southern segregationists and anti-New Dealers, working against a Republican Party, with conservative goals of smaller government, fewer taxes, less regulation, and without pesky compromises or goals thwarted by moderate squishes in their ranks.

The pivotal movement in academia was the famous—among scholars, at least—1950 document from the American Political Science Association (APSA), “Towards a Responsible Two-Party System.” Led by renowned political scientist E.E. Schattschneider, the pamphlet made a strong case that American political parties were ineffectual and dysfunctional as they were, and needed to sort ideologically to provide coherence and move policy agendas forward.

The move toward ideological coherence did not start with the APSA missive in 1950. In the Democratic Party, Franklin Roosevelt was regularly frustrated by the Boll Weevils and tried, without success, to punish them at the polls in 1938. As Rosenfeld, a political science professor at Colgate University, notes, FDR also talked to Republican Wendell Willkie about realigning the parties by moving moderate and liberal Republicans over to his side to replace the Southerners. Among Republicans, the struggle for control went at least as far back as the progressive movement, from the early decades of the twentieth century. It was sharpened by the war in the 1940s for the presidential nomination—between conservatives led by Robert Taft and moderates who beat Taft with the Willkie campaign in 1940 and then beat him with Thomas Dewey in 1944 and 1948. This pattern continued with the elections of Dwight Eisenhower and Richard Nixon, before Barry Goldwater finally achieved what Taft could not. If his victory was Pyrrhic in 1964, it set the stage for Ronald Reagan’s much bigger triumph 16 years later. Yet it is important to recognize that Nixon, after Goldwater, also helped pave the way with his infamous “Southern strategy,” designed to use race directly and indirectly to transform America’s electoral map.

Rosenfeld digs deep into the dynamics of both parties, from the initial skirmishes to the many other factors that pushed the parties apart. His research is prodigious. In my first year in Washington, I worked for both Donald Fraser of Minnesota in the House, and George McGovern in the Senate. Fraser was chairman of the Democratic Study Group, the House reform caucus, and co-leader, with McGovern, of a Democratic Party commission that transformed the rules for delegate selection for presidential nominees. I was on the inside as the presidential selection process was being debated, and as House reforms emerged to break the lock that conservative Southerners had on the positions of power, allowing the wealth to be spread to more junior members and more accurately reflect the ideology of the majority of the majority party; to open up the process from the secrecy that had given the Southern barons and their allies in Democratic machines more leverage.

I was back in Washington again after a brief teaching stint in Bologna, Italy, when the post-Watergate class of 1974 provided the impetus and the bodies to put major reforms over the finish line. I also saw the dynamic that was at play, with Vietnam as a catalyst, to wrest control of the presidential process away from conservative and pro-war establishment forces, including Southern powerhouses like John Connally of Texas, party bosses like Chicago Mayor Richard Daley, and union leaders like George Meany. Rosenfeld gets it just right, through prodigious research in the archives, including sifting through countless volumes of historical papers of the participants, and writes about it clearly and compellingly. Another masterful and valuable new book, The Class of ’74, by historian (and longtime House staffer) John Lawrence, provides even richer detail about what that group of activist new members accomplished. It delves into how their reforms dramatically opened up Congress and altered its power rankings, in ways that diminished the role of conservatives, and contributed to the acceleration of realigning change in the South—with a set of consequences not intended by the reformers.

Equally compelling is Rosenfeld’s analysis of the GOP as it moved deliberately in the 1970s—after the key role it had played in enacting both the Civil Rights Act and the Voting Rights Act in the 1960s—to exploit racial divisions and both capture and convert Southern Democrats and Wallaceites. This strategy was not so much a move ideologically to the right as it was a ruthless and pragmatic push to corral votes via the race issue. Here he reinforces the work of other excellent historians of the Republican Party, Geoffrey Kabaservice (Rule and Ruin) and Heather Cox Richardson (To Make Men Free).

Of course, Rosenfeld does not stop with the 1970s. He goes on to talk about the role of Newt Gingrich in the ‘80s, and into the ‘90s, the role of Dennis Hastert, Gingrich’s successor as Speaker of the House, and the use of the so-called “Hastert Rule” designed to make the House Republican Party resemble something closer to a parliamentary majority party. Rosenfeld describes it in the following manner: “From 1995 to 2006, Republicans in control of Congress took the centralizing and discipline-bolstering tactics pioneered by Democrats and dramatically expanded their use.” That is an understatement.

While The Polarizers at least recognizes the role of Gingrich, Rosenfeld misses or underplays a key element. Gingrich did far more than centralize power and shape a disciplined GOP. He pushed the party to vote in unison—akin to a parliamentary minority—against every President Clinton initiative in his first two years in the White House, and in so doing set the stage for Gingrich’s successful campaign against Washington in 1994. Over 16 years, from his election to the House in 1978 through his triumphant election as Speaker after those stunning 1994 elections, Gingrich deliberately moved not just to polarize the parties but to tribalize American politics. Through his rhetoric, his provocations of Democrats, his moves to criminalize policy differences through the ethics processes in Congress, his delegitimizing of Congress and Washington, and his recruitment of candidates who both parroted his divisive rhetoric and believed it, Gingrich sharply moved our politics from polarization to tribalism. He was aided and abetted by another phenomenon, the rise of tribal media, especially talk radio, which was amplified dramatically in the late 1980s by the populist reaction to a massive pay raise for members of Congress, judges, and top executive branch officials. An important name missing in the book is Rush Limbaugh, who used the pay raise to rise to right wing talk-radio rockstar status, creating a new form of divisive communication that reinforced the tribalism.

Polarization does not necessarily have to lead to gridlock or deeper dysfunction; given the nature of the American political system, it can lead to compromises across ideological lines. That is what we saw with, for example, the longtime friendship and partnership between liberal Democrat Ted Kennedy and conservative Republican Orrin Hatch. That alliance gave us the Orphan Drug Act and the Children’s Health Insurance Program, among other things. But tribalism—a belief that opponents are not just those with different philosophies of governance, but evil people trying to destroy our way of life—creates a much more destructive dynamic. That’s something we saw with the quixotic shutdown of government in 1995-96, with the subsequent, ill-fated impeachment of Bill Clinton, and now we’ve seen it culminate in the election of Donald Trump and a bevy of tribal enablers in Congress and the media.

The Polarizers is a book meant to describe the historic roots of an elite-driven impetus to create ideologically responsive parties in the United States. As such, it pretty much ends in the year 2000 (there is a six-page coda on the next 16 years). And that is a significant flaw in an otherwise strong history. Remarkably, there is not a single mention of Mitch McConnell, or of “Young Guns” Eric Cantor, Paul Ryan, and Kevin McCarthy. Using a strategy laid out on the night of Barack Obama’s inaugural in January of 2009—expanding on the Gingrich playbook—Republicans in Congress moved to vote in unison against every Obama initiative, to block what they could, and delegitimize what they couldn’t, and then to inflame and exploit populist Tea Party anger in the midterm elections in 2010 and 2014—anger both at Obama and at Washington itself. McConnell, as Republican Senate leader, broke longstanding norms to use the filibuster routinely as a weapon of mass obstruction on both legislation and confirmation votes of judges. The racial inflammation of the “Birther” movement was blithely ignored or tacitly enabled by most Republican leaders, who also refused to condemn or counter false charges that Obama was a secret Muslim. Tribal media, now amplified by social media, added to the anger, the division and the tribalism, as it metastasized from national politics to states and the broader public. It would have been nice to have an additional chapter on this by Rosenfeld, using his supreme skills as a political scientist and historian, but its absence does not undercut the value of his history.

If both parties have become more homogeneous and further apart, the sharper changes by far have come on the Republican side. That includes their breaches in norms, which have grown dramatically worse in both Congress and in the executive branch since Donald Trump became President. Trump’s moves to double down on tribal identity, protected and repeated by Republicans in Congress, will make far more challenging any efforts to ameliorate the dysfunction.

When Tom Mann and I wrote It’s Even Worse Than It Looks: How the American Constitutional System Collided With the New Politics of Extremism in 2012—a work focused both on the mismatch between parliamentary-style “responsible” parties and the American system and on the emerging radicalism of the Republican Party—we dedicated the book to the late Austin Ranney. In the 1950s, he wrote a penetrating and prescient critique of the APSA tract on responsible parties, noting the likely problems that would arise if Schattschneider and his allies achieved their partisan dream—these problems were baked into the context of a system of separated powers and separate elections for House, Senate, and the presidency.

Ranney was right—but it is far worse than he imagined. New media and social media, which thrive on division and tribalism, have joined with corrupt and amoral leaders to corrode the system even more deeply, shattering norms of governance and defining deviancy down by the day. A corrupt campaign finance system set in place thanks to the Supreme Court and the norm-breaking of Republicans on the Federal Election Commission gives immense leverage to plutocrats and ideologues; a $1 million contribution to Trump’s inaugural committee filtered through a shell company by individuals and groups determined to pack the judiciary with ideologues has gotten a huge return on its investment.

Even if America survives Trump and his congressional enablers through an uprising of civil society, it will not alter the fundamentals of the parties or bring back those norms. The few moderate Republicans in office, along with the conservatives oriented toward problem-solving, have dropped to a trace element at best—leaving the more radical elements in charge. The next generation—those now in state legislatures and in party posts—are more radical than the radical Freedom Caucus. A new generation of polarized and polarizing judges of the Neil Gorsuch/Samuel Alito variety will be expanded to appeals courts and district courts, altering those for decades.

As Rosenfeld aptly documents, both parties had strong and persuasive advocates pushing them to become ideologically coherent entities. Neither party has behaved impeccably—politics ain’t beanbag, as Mr. Dooley noted long ago—and power can and does corrupt. But there has been, as he also admits, a sharp asymmetry to the polarization. And Democrats, if they regain some measure of power in November, will have tough choices to make on how they conduct themselves in the legislative process; whether they respond to the radicalization of the GOP, as they have resisted doing so far, with a sharp move to the bombastic left. The upcoming elections, at both federal and state levels, are pivotal in determining whether our downward slide continues or even accelerates, or is kept in check. But even the most dramatic wave for Democrats will not move us back to big tent parties or to a model of functionality. We face a long and winding road ahead.