Thursday night’s Democratic presidential debate began with a dispute over health care policy and ended with an argument about President Barack Obama’s legacy.

But in each of those moments, and at several points in between, Hillary Clinton and Sen. Bernie Sanders (I-Vt.) were really arguing over something very basic.

Can the next president eliminate, or at least weaken substantially, the political obstacles that block a liberal agenda?

Sanders thinks the answer is yes. Clinton thinks the answer is no.

At the start of the debate, hosted by PBS NewsHour, Sanders made his now-familiar case for replacing existing private and public insurance plans with one government-run program. Such systems work really well overseas, Sanders said, so there’s no reason such a system can’t work here. The big impediment, he said, would be lobbying from the health care industry, because drug and insurance companies would perceive such a plan as a threat to their profits.

“If -- and here's the if -- we have the courage to take on the drug companies, and have the courage to take on the insurance companies, and the medical equipment suppliers, if we do that, yes, we can guarantee health care to all people in a much more cost effective way,” Sanders said.

Clinton, in her portion of the discussion, made some of the same arguments she has before -- suggesting, among other things, that the Sanders plan would be far more expensive than he has claimed. But she really stressed another point -- that enacting health care reform is difficult. Getting the Affordable Care Act through Congress required a massive, politically draining effort -- and it very nearly failed. Starting a new fight now, she said, would probably not work and might even backfire.

It’s a lesson she said she learned firsthand in the 1990s, when she was first lady and in charge of President Bill Clinton’s failed attempt to pass health care reform then. “You know, before it was called Obamacare, it was called Hillarycare,” Clinton said. “And I took on the drug companies and I took on the insurance companies to try to get us universal health care coverage. And why I am [such] a staunch supporter of President Obama's principal accomplishment -- namely the Affordable Care Act -- is because I know how hard it was to get that done.”

The end of the debate got a little nastier, as Clinton cited a few recent instances when Sanders appeared to be making statements critical of Obama and his accomplishments. Clinton noted that Sanders had given a favorable blurb to a book called Buyer’s Remorse: How Obama Let Progressives Down. She also cited an interview, which aired on MSNBC on Thursday, in which Sanders implied that Obama hadn’t done enough to rally the grassroots behind his causes.

“This is not the first time that he has criticized President Obama,” Clinton said. “And I just couldn't ... disagree more with those kinds of comments. You know, from my perspective, maybe because I understand what President Obama inherited, not only the worst financial crisis, but the antipathy of the Republicans in Congress, I don't think he gets the credit he deserves for being a president.”

Sanders responded angrily, saying that Clinton had taken his comments out of context -- and making clear that, of course, he admires and supports the president. But Sanders also acknowledged that he sometimes has been critical of Obama. “I have voiced criticisms,” Sanders said. “You're right. … But ... you know what the blurb said? The blurb said that the next president of the United States has got to be aggressive in bringing people into the political process. That's what I said. That is what I believe.”

The difference here is real, if subtle. Sanders, in his speeches and campaign material, spends very little time talking about what was done during the Obama presidency. His focus is almost entirely on what he wants to do next -- about how a grassroots movement can deliver programs like government-run health insurance, free public college tuition, and a minimum wage that goes all the way up to $15 an hour. He praises Obama, but he is also promising something new.

Clinton has said she supports the same goals -- making sure everybody has health insurance, more affordable college tuition, a higher minimum wage. But she spends a lot more time touting what Obama has done and pledging to defend it against Republican attacks. She casts herself as the heir to Obama, as somebody who will carry on the crusade he started.

Sanders’ argument is a lot more inspiring. It promises big changes -- a revolution, literally. It suggests that progressives can realize their goals if they only put in enough effort, and build a coalition large enough to overcome the power of special interests. It’s precisely the kind of message that resonates with idealists, which probably helps explain why Sanders has been such a hit on college campuses and with young voters.

But Sanders’ vision also requires a big leap of faith. While mass movements have changed American politics before -- the Civil Rights movement comes quickly to mind -- it’s taken years and sometimes decades of organizing and fighting. Even then, progress was slow, painful, and disheartening to many of its proponents.

Clinton’s argument, by contrast, isn’t likely to excite anybody. It amounts to saying that the future is limited, rather than limitless -- and that best hope for the next four years is to avoid backtracking, rather making great strides forward. That’s not a lot of comfort to people struggling with tuition or medical bills, or unable to find a good job.

But Clinton’s argument also recognizes that money in politics isn’t the only obstacle to sweeping change. Progressives also face tough resistance from conservative ideologues -- starting with the ones who run Congress -- and would need to win over a public that is frequently ambivalent about, or even hostile to, big government initiatives. And that’s to say nothing of prejudices, like sexism and racism and anti-immigrant sentiment, that make progressive policies even more difficult sell. Clinton wants to overcome these obstacles, but she believes it's going to be a long-term project -- with mostly incremental victories for now.

Just like Sanders' pitch is a natural fit for his constituency of younger voters, the Clinton argument probably resonates with older voters who have lived through the same political battles she has. They might like what Sanders is promising. They just don't believe he can deliver it.

PHOTO GALLERY Where Democratic Candidates Stand On Immigration