Despite months of preparation, Clinton appeared openly shocked by the direct and deliberate performance of the self-declared independent socialist, who repeatedly won over the audience in attendence with his ambitious vision and sharp defense of an expansive welfare state. It was the first time that many viewers had ever seen Sanders - and for many, the first time they had seen an American socialist.
"He just made sense," said Ahmed Parmar, an auto mechanic from Fairfax, Virginia. "I do not know why Hillary didn't have better answers for him. It made me question her on a lot of issues."
Sanders advanced with a multi-pronged attack that left Clinton scrambling and often flustered for most of the evening, touching on everything from the invasion of Iraq to breaking up so-called "too big to fail" banks.
"Congress does not regulate Wall Street," Sanders said in one typical moment, responding to Clinton's call for stronger financial regulations. "Wall Street regulates Congress. Going to them and saying 'please do the right thing' is kind of naive."
That conception of the government's relationship to the market has long been a central component of socialist political theory, and one that Americans voters, particularly after the 2008 financial crisis, have found increasingly persuasive. But Clinton, whose entire political career has progressed well within the confines of liberal-centrist capitalist orthodoxy, seemed entirely unfamiliar with that line of criticism, and struggled to answer it.
"I represented Wall Street, as a senator from New York, and I went to Wall Street in December 2007 - before the big crash that we had - and I basically said, 'cut it out!'" Clinton said.
Clinton will have to hope for a stronger performance during her next debate to slow the momentum of Sanders, who has cut her national lead from an overwhelming 60 points to a shrinking 13 - and crucially, who has taken the lead in several critical early primary states. The Sanders campaign, meanwhile, is pressing forward.
"Bernie Sanders won a major victory in Tuesday night's first Democratic debate, according to polls, focus groups, media experts, independent analysts and social media analytics," the campaign announced in a press release.