Clinton went first. After running back Sanders' attacks on her, MSNBC host Rachel Maddow asked, "Why should liberal Democrats support you and not Senator Sanders?"
The former secretary of state was prepared.
"I am a progressive who gets things done," she fired back. "The root of that word, progressive, is progress."
"I've heard Sen. Sanders' comments," Clinton continued, winking at the roots of the question. "It's really caused me to wonder who's left in the progressive wing of the Democratic Party. Under his definition, President Obama is not progressive because he took donations from Wall Street. Vice president Biden is not progressive because he supported Keystone. Sen. Sheehan is not progressive because she supports the trade pact even the late, great Sen. Paul Wellstone would not fit this definition because he voted for DOMA."
Both rhetorically and in policy proposals, Clinton advocates for legislation she claims is more likely to pass an often gridlocked Congress. Most notably in the current context, she differs with Sanders on how to go about achieving universal health care.
"I don't want to see us start over again with a contentious debate," she said during a past debate and frequently on the trail. "I want us to defend and build on the Affordable Care Act and improve it."
For Sanders, the path is more direct. He wants "Medicare for all." And now.
Progressivism to him is embodied by the "political revolution" -- a mix of economic populism and a beefed up welfare state -- he calls for on the stump and name-checked on the debate stage.
"To my mind, what we have got to do is wage a political revolution where millions of people who have given up on the political process stand up and fight back," he said. "Demand a government that represents us and not just a handful of campaign contributions, contributors.
"Now all the ideas that I'm talking about, they are not radical ideas. Doing, making public colleges and universities tuition free, that exists in countries all over the world, used to exist in the United States," he continued. "What we need to do is to stand up to the big-money interests and the campaign contributors. When we do that we can in fact transform America."
For 15 minutes on Thursday night, the nuts and bolts of particular issues and political scuffles were set aside, allowing the two finalists for the Democratic Party's 2016 presidential nomination to lock horns on the fundamental direction of American liberalism.
It is a question that could come to define what is shaping up to be a longer contest than almost anyone expected.