A few years ago, political scientists and some popular commentators began examining a phenomenon known as "asymmetric polarization," a fancy way of saying that while the two parties were moving apart ideologically, they weren't doing so at the same rate. For a variety of reasons, Republicans were heading faster to the ideological edges. "Republicans are galloping right while Democrats are trotting left," wrote Jacob Hacker and Paul Pierson in their 2006 book Off Center; or as David Roberts put it, "The left's gone left but the right's gone nuts."

There was copious evidence in congressional votes and public opinion to substantiate that assertion, not to mention the entire lunatic phenomenon of the Tea Party, which culminated in the nomination and election of President Trump. But in just the last couple of years, we may have seen a reversal, in which Democrats are the ones who are changing while Republicans aren't, or at least aren't as much. Part of the reason may be that there's only so much room on the right for the Republican Party to go, unless they propose to turn The Handmaid's Tale into a documentary, bring back child labor, and round up all the endangered species for processing into lunch meat. But Democrats are undergoing a much more striking ideological evolution right now, genuinely turning into a more emphatically liberal party.

Are there risks associated with such a change? Sure. But they're actually in a better position to profit from them than Republicans were at their periods of rapid ideological evolution. That's because on nearly all the issues where Democrats are becoming more liberal as a party, they're moving toward public opinion, not away from it. And Republicans, especially President Trump, are making it even easier.

Consider what may be the most dramatic change of late, the Democratic Party's almost complete embrace of what gets reduced to the shorthand of "single payer" health insurance. It would actually be better termed "universal coverage," since there will be some variation in what Democrats support (here's a sample plan if you're interested), but what's important is that just about every Democrat who might run for president in 2020 has already come out and said they support single payer, by which they mean a government guarantee of health coverage for everyone.

This is an enormous change. Ten years ago, all the major Democratic candidates rallied around something like the public/private hybrid Mitt Romney had passed in Massachusetts. Just two years ago, Bernie Sanders offered a single payer plan, which Hillary Clinton said was too radical and proposed enhancing the Affordable Care Act.

But now President Trump and the Republican Congress have effectively ended the argument among Democrats. Their failed attempt to repeal the ACA showed everyone how much the public values Medicaid, while their somewhat successful efforts to sabotage the law will continue to reduce security and increase premiums, creating the space for Democrats to propose something much more ambitious.

And the public looks ready to hear their case: The most recent Kaiser Family Foundation poll found that 59 percent of Americans supported "a national health plan, or Medicare-for-all, in which all Americans would get their insurance from a single government plan." When they changed the question to pose such a plan as open to anyone but people could keep what they have if they wanted, support climbed to 75 percent. Republicans seeing those results were surely aghast.

Democrats are finding support for the other more clearly liberal positions they're taking, too. We've entered a new era of our gun debate, where Democrats now see advocating more restrictions on guns as a help, not a hindrance, in getting elected. They've gone from saying the minimum wage should be increased by a buck or two, to saying it should go to $10.10 an hour, to advocating a $15 an hour minimum. That was considered an extreme idea not long ago, but polls now find a majority of Americans saying they'd support it. A federal job guarantee, in which the government would provide work for everyone who couldn't find it elsewhere, is getting new attention, including from some potential presidential candidates. Many of them will probably come out in favor of marijuana legalization, which is now supported by around two-thirds of the public.

All that isn't to say there aren't still vigorous debates about policy going on within the Democratic Party. But unity is more likely today than it was in years past, precisely because of polarization. As the two parties move apart, what once were ideologically diverse coalitions become more unified. Primary voters wind up putting a premium on ideological purity, and the candidates that emerge are farther from the center.

Which could be a problem if the public doesn't like what you're selling. That is in fact the case for Republicans; much of what they want, like tax breaks for the corporations and the rich, outlawing abortion, and a weaker safety net, is deeply unpopular. They've overcome that challenge through strong mobilization efforts and effective use of racial resentment, but it's something they always have to struggle with — and it's why when they get power they don't even try to enact much of what they believe in.

Democrats, on the other hand, can at least know that the liberal program around which they're coalescing has broad support. Which means that if and when they get full control of the government — say, after the 2020 elections — they'll know exactly what they ought to do.