The American electorate has undergone fundamental demographic changes over the past 20 years that will continue for the foreseeable future. These changes—in particular, the growth of the millennial generation—will result in a significantly different landscape for the two parties in 2024 and have a consequential impact on policy-making and governing.

Some have argued that the growth of groups that are more favorably inclined toward the Democratic Party (nonwhites, millennials, unmarried women, professionals, the nonreligious) will inevitably lead to a long-term Democratic majority. When it comes to millennials, this is true to an extent—but there are important caveats that make the eventual outcome less certain. Events on the ground matter, and the actions taken by the two parties over the next dozen years will exert a heavy influence over how this group votes in 2024.

In 2008, 48 million millennials (those born between 1978 and 2000) were eligible to vote, and 25 million actually did. By 2020, the first year in which all millennials will be eligible, those numbers will about double, to 90 million and 52 million respectively. Millennials will make up more than 35 percent of the electorate in 2024, up from 20 percent in 2008, and about the same percentage of the electorate currently made up by baby boomers.

The 25 million millennials who voted in 2008 went strongly Democratic, giving President Obama 66 percent of their vote and House Democrats nearly as much, at 63 percent. But this support dropped significantly in 2010, to 55 percent. And while other 2008 Obama base groups such as nonwhites and unmarried women have seen their level of support for Obama’s re-election return to 2008 levels after a 2010 drop-off, recent polling shows that millennial voters’ enthusiasm still lags. Fifty-eight percent of these voters self-identified as Democrats in 2008, but that was down to 50 percent at the end of 2011—the largest decline of any age group.

What explains the drop? This group was hit particularly hard by the economic downturn. And while they continue to lean Democratic, their high expectations for Obama combined with a perceived lack of progress on the economy has clearly dented their once overwhelming allegiance to the Democratic Party—at least temporarily. But how is that likely to change over the next 12 years?

There is little doubt that millennials are generally much friendlier to Democrats than Republicans. They are more likely to identify as liberals and are much less religious than older generations. They hold far more progressive views on social issues. Over 60 percent support gay marriage, and they are much more supportive of legalizing marijuana, promoting racial justice, and restricting guns, among other issues. Just as important, millennials are dramatically more supportive of a larger and more activist government (56 percent support a bigger government with more services, versus 41 percent of all adults), and they show the most support of any age cohort for President Obama’s health-care bill, for environmental regulations, and for alternative energy. They are much more supportive of immigration and immigrants, and much less supportive of American exceptionalism in foreign policy.

Not surprisingly then, while these voters continue to evince some frustration with Democrats and Obama, their view of the Republican Party has soured dramatically since 2010 (going from 38 to 50 percent unfavorable) as the GOP’s radical anti-government, anti-gay, anti-contraception Tea Party agenda has come to the fore. Indeed, Pew’s excellent survey from late last year on the generation gap shows that millennials overwhelmingly view the GOP as extreme and unconcerned with people like them.

But millennials present some concerns for Democrats over the long term as well. These younger voters are not at all convinced that Social Security and Medicare will be around when they retire and are therefore much more open to making major changes to the entitlement system, including benefit cuts. Clearly, the Democrats’ central message of 2012—tying Republicans to Representative Paul Ryan’s very unpopular Medicare overhaul—is not something Democrats will be doing in 2024 and beyond.

So where does that leave the political and policy landscape of 2024? If they want to have any hope of appealing to millennials, Republicans will be forced to moderate their positions on social issues significantly. By 2024, millennials will likely have taken the issue of marriage equality off the table; its ascension in America is inevitable. Similarly, policy proposals like the Blunt Amendment or so-called personhood amendments that would restrict access to contraception will become something close to political suicide. Meanwhile, the prospects for bipartisan immigration reform by 2024 are very strong, as Republicans will also have to moderate their views on immigration to prevent alienating both millennials and the equally fast-growing Hispanic demographic.

But the Democrats will face challenges, too, notably on Social Security and Medicare. By no means do millennials support the kind of dismantling of Medicare and Social Security that House Republicans have proposed; in fact, in a just-released survey from Democracy Corps, only 31 percent of millennials support the new Ryan budget. However, by 2024 Democrats’ ability to use cuts to these programs as a political bludgeon will have likely been attenuated. This, in turn, will likely open the door to some significant changes to the entitlement system.

For nearly half a century, the unyielding defense of entitlement programs has been a constant of the Democratic Party. But these demographic shifts will force Democrats to confront a reality in which they can’t use Social Security and Medicare as their chief justifications for a more activist government. Their ability to find alternative justifications that speak to the everyday needs of millennials may determine if these voters continue to trend Democratic.