Bloomberg via Getty Images The debate between Hillary Clinton and Bernie Sanders has been simmering on the left for most of the Obama presidency.

The debate Bernie Sanders and Hillary Clinton are having in the Democratic presidential campaign is in many ways about the future. It's about their different theories of political change, and about how to get shit done.

Sanders, the independent senator from Vermont, is unabashedly ambitious. As president, he’d try to stretch the boundaries of the politically possible, rallying grassroots support to force sweeping liberal initiatives through Congress. Clinton, the former secretary of state, is part of the political establishment and lives by its conventions. If elected, she would slog her way through the usual political muck, hoping to realize a progressive vision through a series of hard-fought, incremental victories.

But lurking behind this argument about the future is a dispute that's really about the past. It’s a debate over what Obama accomplished in office -- in particular, how significant those accomplishments really are. And it's been simmering on the left for most of the last seven years.

On one side of this divide are activists and intellectuals who are ambivalent, disappointed or flat-out frustrated with what Obama has gotten done. They acknowledge what they consider modest achievements -- like helping some of the uninsured and preventing the Great Recession from becoming another Great Depression. But they are convinced that the president could have accomplished much more if only he’d fought harder for his agenda and been less quick to compromise.

They dwell on the opportunities missed, like the lack of a public option in health care reform or the failure to break up the big banks. They want those things now -- and more. In Sanders, they are hearing a candidate who thinks the same way.

On the other side are partisans and thinkers who consider Obama's achievements substantial, even historic. They acknowledge that his victories were partial and his legislation flawed. This group recognizes that there are still millions of people struggling to find good jobs or pay their medical bills, and that the planet is still on a path to catastrophically high temperatures. But they see in the last seven years major advances in the liberal crusade to bolster economic security for the poor and middle class. They think the progress on climate change is real, and likely to beget more in the future.

Lurking behind this argument about the future is a dispute that's really about the past.

Their priority in this election is choosing a president focused not just on advancing this agenda where possible, but also on defending the Obama legacy from Republican assault. And that’s what Clinton is promising them.

The distinction is subtle, but it's there once you start looking. Check out the Sanders campaign website, and you’ll find a collection of serious papers on domestic policy issues, laying out comprehensive and frequently audacious plans. But many mention Obama and his legacy only in passing, or not at all.

One exception is a page on health care, which dutifully recounts the achievements of the Affordable Care Act. Then it quickly brushes past that to focus on the problems of the remaining uninsured and those with coverage who face staggering premiums and out-of-pocket expenses. The health care page doesn’t mention resisting GOP efforts to repeal the law, just as a page on climate says nothing about blocking Republican attempts to roll back the new Environmental Protection Agency regulations on emissions.

Clinton’s website is very different. On the health care page, she vows to “defend the Affordable Care Act (ACA) against Republican efforts to repeal it” before sketching out her own, more limited ideas for reform. On her climate page, she vows that protecting existing regulations will be a “top priority.”

The statements are consistent with the rhetoric of her new Iowa ad, which -- as columnist Jonathan Chait noted recently in New York magazine -- promises to “stop the Republicans from ripping all our progress away” while clips of Donald Trump and Ted Cruz play on the screen. You'll see nothing like that in the Sanders Iowa spots, which focus on his sweeping plans for the economy and health care -- or show images of his energetic, idealistic supporters set against a Simon and Garfunkel soundtrack.

The contrast between Sanders and Clinton was also apparent on Monday evening, when the two candidates made separate appearances at a CNN town hall in Des Moines, Iowa. Sanders answered an early question about his agenda by focusing on all the problems that America faces -- stagnant wages, unaffordable health care, too much power on Wall Street -- and calling for a political revolution to address them. “This is not what America is supposed to be about,” Sanders said. “Establishment politics is not good enough. We need bold changes.”

It was a version of Sanders’ stump speech, and it was a stark contrast to Clinton. Reprising one of her own favorite lines, the former secretary of state talked about continuing the “progress we had under President Obama” and her commitment “to do everything we possibly can not to let the Republicans rip it away and turn us backwards.”

To be clear, these differences of emphasis belie the relatively similar views that Sanders and Clinton have when it comes to what America should really look like. Both are committed progressives. Both want government playing an active role in guaranteeing economic security, setting rules for the economy, protecting traditional victims of discrimination and preserving the environment. Both think Obama advanced this agenda, but that there is still more to do.

As for their differences over strategy and tactics, they might not even matter much if either becomes president. Republicans are likely to control at least one house of Congress for the next four years, severely limiting the opportunities to pass ambitious legislation. If a Democratic president succeeds Obama, he or she will spend much of the next four years stopping a lot of GOP-backed legislation from becoming law. Both Sanders and Clinton would perform that task energetically.

But their different attitudes towards the Obama era could still have political significance. Supporters of Sanders would argue -- and Sanders presumably believes -- that his pitch better fits the mood of the electorate, given that polls consistently show majorities of Americans believe the country is on the “wrong track” and large numbers of people really are struggling with stagnant wages and high medical bills. Running so heavily on the Obama record sure makes less sense when that record doesn't excite the public.

In this view, voters within and outside the Democratic Party are craving a radical change, something more sweeping and transformative than anything Obama achieved. And while enacting such an agenda may not be politically possible in the next two years, Sanders can use the inevitable Republican resistance to rally the public behind Democrats in the 2018 midterms -- a point a top Sanders adviser made to The Huffington Post's Sam Stein earlier this week. In the meantime, Sanders supporters, he will be generating support for a much more liberal agenda, and setting the terms of debate in a way that pushes legislative compromises further to the left. It's not a crazy argument.

But ignoring or denying the progress of the Obama era also carries risks, as Clinton supporters point out. Republican politicians and their allies in the conservative movement speak in one consistent voice, arguing that the Obama agenda has been not just a failure, but a catastrophe. Absent a strong defense of these programs, average voters may listen and conclude that his programs must not have done much good. In the long run, that can undermine the public’s faith in government activism of any kind. It can also set up liberals for a perpetual cycle of disappointment, as they discover over and over again that the compromised progressive reforms coming out of Washington don’t match their lofty expectations.

Of course, ambivalence towards Obama-era achievements can have a more immediate effect, too. They can make those programs politically vulnerable, to the point where Republicans have not just the institutional power but also the political cover to dismantle them. If that happens -- if millions of people lose health insurance, if progress on climate change halts or reverses -- the progressives underwhelmed by the last seven years might decide they made a big difference after all.