The debate within the Democratic primary can be boiled down to this: Bernie Sanders is calling for a revolution. Hillary Clinton is defending the current regime.

That debate, long lurking below the surface in the Democratic contest, was displayed more clearly than ever in tonight's primary debate, which feature Clinton and Sanders repeatedly clashing over whether to build on President Obama's legacy or to cast it aside in favor of more radical change.

This divide was particularly apparent in the back-and-forths over health care and financial reform. The Sanders and Clinton camps have been arguing all week over the merits of single-payer health care, with Sanders making the case that only a single payer system can truly provide the sort of guaranteed universal coverage that Democrats have long promised, and Clinton arguing that even attempting a single-payer system would jeopardize all of the gains that Democrats have made with Obamacare.

Asked about the squabble, Clinton said that with Obamacare, "we finally have a path to universal health care…I do not to want see the Republicans repeal it, and I don't to want see us start over again with a contentious debate. I want us to defend and build on the Affordable Care Act and improve it."

Sanders responded by accusing her of ducking the question—tonight, and, implicitly, throughout the week. "What a Medicare-for-all program"—his shorthand for single-payer—"does is finally provide in this country health care for every man, woman and child as a right." Throughout the night, Sanders rejected Clinton's incrementalism in favor of pursuing goals that were both more immediate and more grandiose.

The same divide was apparent on the subject of financial reform, where Clinton defended the Dodd-Frank regulatory overhaul signed under president Obama, and hit Sanders for not being more supportive of the current administration.

"Where we disagree," she said, "is the comments that Senator Sanders has made that don't just affect me, I can take that, but he's criticized President Obama for taking donations from Wall Street, and President Obama has led our country out of the great recession. Senator Sanders called him weak, disappointing….Now, I personally believe that President Obama's work to push through the Dodd- Frank… [LAUGHTER] The Dodd-Frank bill and then to sign it was one of the most important regulatory schemes we've had since the 1930s. So I'm going to defend Dodd- Frank and I'm going to defend President Obama for taking on Wall Street, taking on the financial industry and getting results."

I'm going to defend President Obama. Those were not only the most important words in Clinton's answer on the topic of financial reform, but the most important and telling words she uttered at any point during the debate. Clinton's entire argument for her presidency in the primary is that she will defend President Obama, and build upon his legislative and political victories. She is explicitly the candidate of the status quo.

And Sanders is, just as explicitly, the candidate of something bigger, something more dramatic, something that goes far beyond what he views as the timidity and smallness of the current administration—and, arguably, of American politics.

"This campaign is about a political revolution to not only elect a president but to transform this country," Sanders said early in tonight's debate. The word "revolution" came up several times during tonight's event, and it is key to his pitch to Democratic voters who are frustrated, disappointed, and even angry about the state of affairs at the end of the Obama presidency. He is promising something wildly different, in contrast to Clinton's promise of more of the same.

The biggest problem for Clinton is that, as a glimpse as the polls, which show Sanders running close to or ahead of Clinton in both Iowa and New Hampshire, indicates, many Democratic voters clearly long for the revolution that Sanders promises. The biggest problem for Sanders is that the revolution is easier imagined than accomplished. Asked about the failure of single-payer health care in his home state of Vermont due to the burdensomely high tax increases that would have been necessary, Sanders essentially dodged the question, but effectively admitted that his plan would require higher taxes on most everyone, including the middle class.

And that, in some sense, is the essence of the debate within the Democratic party primary—whether to throw in with am expensive, vague fantasy that has no real chance of success, or whether to accept a thoroughly disappointing status quo.

Martin O'Malley was also at tonight's debate, and he frequently reminded people of his time as governor of Maryland.