That meant that Sanders — now a real threat for the Democratic presidential nomination — was sure to face heightened scrutiny at the University of Wisconsin-Milwaukee face-off, with tough questions about how he'd carry out his promised "political revolution" and deliver on sweeping promises for greater economic equality.
Sanders rose to the occasion.
Leveling some of his sharpest attacks on Clinton to date, the senator from Vermont not only showed the instincts necessary to mount a winning campaign for the nomination; his message of economic populism and anti-establishment anger also set the terms for much of the PBS-hosted confrontation, underscoring how his insurgent campaign has helped reshape the Democratic Party.
A new dynamic: Nowhere were the new contours of the Democratic race more readily apparent than in Clinton's opening statement. Seeking to tap into the populist sentiments that Sanders has harnessed, Clinton decried a "rigged" economy and said that Americans were rightfully "angry" — even suggesting that she'd "go further" than the democratic socialist in confronting those challenges.
"I know a lot of Americans are angry about the economy. And for good cause: Americans haven't had a raise in 15 years. There aren't enough good-paying jobs, especially for young people. And yes, the economy is rigged in favor of those at the top," Clinton said.
"We both agree that we have to get unaccountable money out of our political system and that we have to do much more to ensure that Wall Street never wrecks Main Street again," she continued. "But I want to go further. I want to tackle those barriers that stand in the way of too many Americans right now."
Clinton's acknowledgment of popular anger was a sharp turnabout from her rhetoric before Sanders defeated her in New Hampshire by 22 percentage points.
"I know that it's maybe not the most appealing or charismatic message to say, 'Hey, guys, be angry, and then let's roll up our sleeves and get to work,'" Clinton said at a Saturday rally before the primary. "Anger is a powerful emotion, but it's not a plan."
Of course, Sanders has offered both anger and plans, and Clinton struggled to find her footing in criticizing his policy proposals on Thursday night.
Renewing her attacks on Sanders' single-payer health care plan, Clinton simultaneously argued that more steps should be taken to insure millions of Americans still left uncovered under the Affordable Care Act, as Sanders advocates, and that "the last thing we need is to throw our country into a contentious debate about health care again." It's not clear how one can achieve the former without engaging in the latter.
And Clinton risked finding herself further out of step with the Democratic base by arguing that Sanders' Medicare-for-all plan, by phasing out the private insurance system, would be too disruptive and leave many Americans worse off. A December 2015 Kaiser poll found 81% Democratic support for a Medicare-for-all system.
Going on the offensive: Such forceful attacks from Clinton hardly represent a new development in the Democratic debates. What changed on Thursday night, though, was that Sanders showed a new willingness to depart from his tendency to treat the debates as another opportunity to recite his anti-inequality stump speech while occasionally responding to jabs at his record. In the clearest sign yet that he's in the race to do far more than make a point, Sanders waged a hard-hitting offensive against Clinton, catching her off guard at crucial moments.
After Clinton ridiculed the idea that her contributions from Wall Street and other corporate interests compromised her integrity, Sanders was ready with a caustic response.
"Let's not insult the intelligence of the American people. People aren't dumb," he shot back. "Why in God's name does Wall Street make huge campaign contributions? I guess just for the fun of it! They want to throw money around."
Sanders didn't stop there, calling into question Clinton's progressive bona fides on issues like trade agreements, Social Security expansion and foreign policy.
Clinton seemed particularly rattled when Sanders noted her support in 2014 for deporting Central American children who fled to the United States in the face of gang violence in their home countries — a stance that could complicate Clinton's efforts to depict herself as an ally of immigrant advocates. In an awkward response, Clinton suggested she supported the deportations to "send a message."
"I made it very clear that those children needed to be processed appropriately. But we also had to send a message to families and communities in Central America not to send their children on this dangerous journey in the hands of smugglers," she said.
Speaking with reporters after the debate, Sanders campaign manager Jeff Weaver blasted that response, saying Clinton would have treated migrant children "like a package marked 'return to sender.'"
In a strike that both exploited left-wing concerns about Clinton's hawkishness and depicted her as a creature of the establishment, Sanders assailed her warm relationship with Henry Kissinger, whom the senator called "one of the most destructive secretaries of state in modern history."
"I'm proud to say that Henry Kissinger is not my friend," Sanders said.
Clinton resisted being baited into a full-throated defense of the controversial Nixon-era diplomat, but she did laud his work to forge a closer relationship between the U.S. and China.
Far from flawless: While Thursday night showcased Sanders at his punchiest yet, his performance wasn't without shortcomings.
With the contest now shifting toward states like South Carolina with higher proportions of African-American voters, the candidates devoted much of the evening to discussing their plans for criminal justice reform. In one of his boldest pledges yet, Sanders pledged that the U.S. would no longer have the largest incarcerated population in the world by the end of his first term.
But in seeking to make inroads with black and Latino voters, Sanders could have called Clinton to task for her support — reiterated in last week's debate — for capital punishment, which disproportionately targets nonwhites. Instead, he let that opportunity pass.
"We've not really engaged in a lot of attacks," Weaver told Mic after the debate, despite Sanders' pointed jabs at Clinton throughout the evening. Asked why Sanders didn't seize the chance to highlight a key difference on criminal justice policy, Weaver said, "I just don't think [the death penalty] came up tonight."
It did not — but if the progressive reception to Clinton's stance on the issue is any indication, it's a topic Sanders had every incentive to insert into the debate himself.
Meanwhile, Clinton continued to far outpace Sanders on foreign policy questions. Cognizant that he must do more than simply remind voters that, unlike Clinton, he opposed the Iraq War, Sanders has formulated more comprehensive critique focused on her readiness to support regime change. But in running down a laundry list of U.S.-backed interventions, name-checking figures like the deposed Iranian leader Mohammed Mossadegh, Sanders sounded more like a bright sophomore who'd just discovered Noam Chomsky than a strategic thinker about the role the U.S. should play in the world.
Indispensable as a foreign policy vision is for a president, international affairs rank low on Democratic voters' list of priorities this year. Far more prominent are issues like wages and the economy — where Sanders' progressive populism captures the new energy pulsating through the Democratic Party. Long averse to fiery appeals, and not given to bashing the economic and political elite of which her family is indisputably a part, Clinton is struggling to navigate that new climate — and Thursday night was a stark reminder.