In a recent New York Times story devoted to the alleged impatience of New York City Mayor Bill de Blasio’s progressive constituency with the pace of his attack on inequality, New York University’s highly quotable professor of urban policy and planning, Mitchell Moss, explained why the mayor should simply ignore the issue. “Inequality,” he explained, “is not something that a mayor of New York can solve.” What’s more, he added, “You can never make the liberals happy. Because when you ask them what they want, the answer is always: M-O-R-E.”

Moss is not entirely wrong. It’s true that “inequality” can never be solved, least of all by a mayor. But Moss’ second argument rankles. One wishes it were true. But if anything, liberals are known for making too few demands and not holding Democrats accountable enough. Has Barack Obama paid a significant price among liberals for his Bush-like extension of presidential power? Or for his failure to push a single-payer health system? And by the way, does Hillary Clinton look as if she is worried about how difficult it will be to satisfy the demands of the Democrats’ liberal base, with her neocon-ish foreign-policy pronouncements and her Wall Street-friendly economic approach?

It should be obvious—at least it feels obvious to me—that the only way to get progressive measures accomplished in America is as a liberal working within the system. America is not, and never has been, on the cusp of revolutionary change. It is, as has been frequently pointed out, a conservative-minded country in pursuit of liberal goals. However, liberals have too frequently shown a willingness to grow overly comfortable with the conservative part of that equation. They need to be shaken up occasionally, and reminded why it is they are making all these necessary compromises in pursuit of the vision that animated them in the first place. And that is why we need radicals. It’s just that we need them to do certain things and not do certain other things.

A leftist agenda of eliminating discrimination, expanding the rights of all Americans, and building an economic and political order based on economic equality makes progress, as the historian Michael Kazin points out, when leftists “win over a section of the governing elite.” That’s where liberals come in, as infuriating as it may be to the radicals who see their often utopian vision compromised and their revolutionary rhetoric reduced to clichéd campaign slogans.

Constructive radical critiques serve two primary purposes: They provide a vision for the future, and they remind liberals not to get too comfortable with the here and now. Milton Friedman, writing with his wife, Rose, in their 1980 book-length manifesto, Free to Choose, makes much of the long-term success of the Socialist Party platform of 1928. The Friedmans reprint the entire platform, together with comments jealously judging the relative success of the New Deal, the Fair Deal, and the Great Society. Would liberal Democrats have been able to cause Milton and Rose so much tsuris without the Socialists laying out a vision that in 1928 looked to be a pipe dream? It’s a counterfactual question, of course, but one that almost certainly answers itself.

Cultural liberalism is clearly triumphing in America today, thanks in significant measure to a constructive alliance between liberals and radicals on issues related to civil rights, women’s rights, and gay rights—between (often) radical movement leaders and (usually) liberal politicians who ultimately do the deals necessary to pass the legislation or pursue the legal battles that lift barriers and deliver change.

Conversely, however, economic liberalism and left-wing radicalism have been in retreat for nearly a half-century (thanks in no small measure to Friedman and the billions of dollars that went into converting so much of the public mind to his conservative economic assumptions). Even so, it’s not difficult to identify examples where radicals have served liberals—and therefore the country—well, with a combination of long-term vision and constructive criticism.

The most obvious example in recent times is undoubtedly Occupy Wall Street, which, with its uncompromising vision and willingness to ignore the niceties of mainstream discourse, forced the issue of inequality to the center of the American agenda. It took a liberal like de Blasio to turn this issue into a winning election campaign for the mayoralty of New York City, but again, it is hard to imagine his “Tale of Two Cities” campaign enjoying anything like the success it did without Occupy’s searing of the “99 percent” into so many New Yorkers’ consciousness.

Along the same lines, albeit on a smaller scale, Zephyr Teachout’s recent campaign for governor of New York state also served to highlight the issue of Andrew Cuomo’s tolerance—one might even say encouragement—of corruption in a way that would have been lost without her campaign. Teachout is absolutely correct to claim, “We are at the beginning of a serious debate about what the Democratic Party really stands for, and we would like to think we have helped to start that discussion.”

The fact that she did not come close to winning is less important than the fact that she badly compromised Cuomo’s hopes of running for higher office some day, while at the same time creating the foundation of a potentially powerful political coalition. As Washington Post columnist Harold Meyerson observed in the primary’s aftermath, “In blue cities and states, being a social liberal will no longer suffice.” In the future, Democrats “will have to embrace more far-reaching egalitarian reforms if they are to create again the kind of broad prosperity that was their calling card to the U.S. electorate. A thriving Democratic Party will require more Zephyr Teachouts.”

A third recent example of a constructive radical intervention against liberal complacency can be found in Ta-Nehisi Coates’s recent Atlantic article arguing for reparations to be paid to African Americans for centuries of government-sanctioned discrimination. I must admit that as a responsible pragmatic liberal—and one who sees class as a far more useful organizing tool than race—I was initially confused by Coates’s essay. What in the world was the point of devoting so much energy and argument to something that even its author admitted was impossible “in this universe”? Why not focus on something that might actually be accomplishable, like, say, a rise in the minimum wage, or job protections for involuntary part-time workers?

But the reaction to Coates’s piece more than demonstrated its value. As Jelani Cobb explained in a New Yorker blog post, “[T]he conversation Coates has restarted is not really about reparations. It is, more fundamentally, about acknowledging the bastard history that would warrant reparations in the first place…. The point of Coates’s essay—and, ultimately, the point of this conversation, despite the political impossibility of enacting reparations—is a broader understanding of black poverty as the product of public policy and private theft facilitated by racism.” None of us can know to what degree, or in what form, this knowledge may one day be turned into power. But liberals, by having to focus on the evidence Coates brought to light, will certainly be both more motivated and more sophisticated in the manner they pursue racial and economic justice for the ongoing victims of America’s “original sin.”

This is what constructive radicalism looks like. It can be critical, no doubt, but it creates a foundation upon which a better liberalism might be built. But there’s another kind, too. There’s a leftist radicalism that turns bitter over the frustration with its own irrelevance and hence acts out its identity through destructive and counterproductive campaigns that not only fail to advance progressive goals, but actively retard them. Ralph Nader’s 2000 presidential campaign was perhaps the ne plus ultra of what Lenin once (in a decidedly different context) named an “infantile leftist disorder,” but one can see examples of it all around us. Echoing those radicals who called for the election of Richard Nixon in 1968 under the presupposition that once conditions worsened sufficiently, Americans would “wake up” and see the wisdom of the radicals’ critique, Nader and his followers helped to give us a Bush presidency with all its collateral damage and nothing of significance upon which to build a better future.

A second example of destructive radicalism can be seen in the Boycott, Divestment, and Sanctions (BDS) movement directed against Israel. Because the movement’s leaders refuse to grant the legitimacy of Israel’s existence, it has literally no chance of ever becoming a majority movement in this country, where Israel remains quite popular. Nor does it have much hope of reaching anything but the tiniest sliver of Israel’s Jewish population, since not a great many are eager to support a program that calls for the destruction of their country.

What the BDS movement does accomplish, however, in its own small fashion, is to isolate, among progressives and some academics, those Israelis who are committed to the only solution that might bring peace and dignity to the people about whom the movement professes to be concerned: the Palestinians living under Israeli occupation in the West Bank and Gaza, and under discriminatory conditions in Israel. It also helps to convince Israelis and their American Jewish supporters that compromise is pointless since the ultimate aim of the Palestinians and their putative political champions is what the Israeli right has always claimed it to be: the destruction of the state and the expulsion of its Jews. This will undoubtedly lead to further repression and immiseration of the Palestinians and destroy what little chance currently exists for a solution that allows them to live in peace and dignity alongside Israel. Hence these are “radicals” whose rhetoric is in almost perfect contradiction with its predictable real-world effects.

This distinction, one must admit, is not always easy to identify in advance, but one can usually glean a pretty good idea by asking what the result will be if one fails to achieve one’s goals any time soon. Choosing between enlisting in the Communist Party or the Socialist Party in the late 1920s and early 1930s may have looked like a more difficult choice for radicals back then than it does in retrospect; after all, the world economy was collapsing, fascism was rising, and the extent of Stalin’s crimes against humanity remained a well-kept secret. Today, however, it serves as a clear guide to the kind of choices that radicals must face. The United States was never going to undergo a communist revolution, thank goodness, and the communists, with their dishonest tactics and overheated rhetoric, retarded progressive goals in this country until they finally (all but) disappeared. But the reformist goals of the socialists, while mocked by those to their left, gave Milton and Rose Friedman, along with their ultra-right allies, nightmares for half a century—and were eventually carried out by a divided Democratic Party.

Much has changed in American liberalism since the New Deal, but nothing quite so much as the loss of its fighting spirit. “I welcome their hatred,” bragged the self-described “militant liberal” Franklin Roosevelt of the “economic royalists” who sought to retain a status quo that operated by and for the wealthy at the expense of everyone else. Radicals of the day helped sustain some of that spirit, as well as planting many of the ideas that FDR and others helped bring to fruition. Our not-so-militant liberals of today could damn sure use some of that kind of help.