Clashing one-on-one for the first time, in a Thursday night debate a day after a New Hampshire town hall, the more than two hours of jostling — over healthcare, Wall Street regulation, what it means to be a true progressive — came down to a fundamental question:
Do Democrats want ground-shaking change after eight years in control of the White House, as Sanders promises, or mere refinement of the programs and policies that President Obama put into place, as Clinton suggests?
"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," Sanders, Vermont's independent senator, declared.
Clinton suggested that Sanders was promising far more than he could deliver. "Let's go down a path where we can actually tell people what we will do," she said. "A progressive is someone who makes progress. That's what I intend to do."
The debate at the University of New Hampshire in Durham fell three days after Clinton scratched out an exceedingly narrow victory over Sanders in the Iowa caucuses and five days before their next test, New Hampshire's first-in-the-nation primary.
The fact the session took place at all was a reflection of the changed nature of the contest. Originally, Clinton agreed to just six debates sanctioned by the Democratic National Committee, which has weathered criticism it tried to shelter the party's front-runner and stave off a serious challenge.
Her willingness to join Sanders onstage — and agree to later debates in Michigan and California — was just one sign the race has grown much tougher than Clinton and her supporters had hoped.
The debate, hosted by MSNBC, featured a longer and deeper discussion of issues, even if many of them were touched upon in earlier debates.
There was also a sharper edge to many of the exchanges, especially when Clinton accused Sanders of impugning her personal integrity by citing the money the former New York senator raised in contributions and speechmaking fees from Wall Street.
Accusing Sanders of wielding "innuendo and insinuation," Clinton contended that she had never "been bought" and was as vigorous as anyone when it came to regulating the financial industry and warning against its excesses. "I think it's time to end the very artful smear that you and your campaign have been carrying out," she said as a chorus of cheers and jeers rose from the partisan crowd.
Sanders, eyes wide and his voice rising, countered that there was a reason that Wall Street, major pharmaceutical companies and other special interests wield enormous political power, and suggested that anyone who denied it was naive.
Noting that not a single Wall Street executive was jailed for wrongdoing associated with the Great Recession, he said, "That is what power is about, that is what corruption is about, and that is what has to change in the United States of America."
Another back-and-forth, over healthcare, spoke to the candidates' competing visions.
Sanders renewed his call for a government-run system providing coverage for every American, similar to that in many countries. "They are spending significantly less per capita on healthcare than we are," Sanders said. "So I do not accept the belief that the United States of America can't do that."
Clinton insisted that Sanders' "numbers just don't add up" and warned that his proposal would plunge the nation into a debate that would undercut the expansion of healthcare to millions of Americans under the Affordable Care Act, also known as Obamacare. "I don't want to rip away the coverage that people already have," she said.
Sanders bristled. "The idea that I would dismantle healthcare in America while we're waiting to pass a Medicare-for-all is just not accurate," he said.
At another point, Clinton vociferously rejected Sanders' assertion that she was not a genuine progressive or, put another way, sufficiently liberal.
"Under his definition, President Obama is not progressive because he took donations from Wall Street," Clinton said, adding that Vice President Joe Biden would be disqualified for supporting the Keystone XL pipeline and the late Sen. Paul Wellstone of Minnesota, a revered liberal, for opposing same-sex marriage.
She then ticked off instances of Sanders opposing gun controls and immigration reforms to question the senator's liberal credentials.
"We could go back and forth like this," Clinton concluded, "but the fact is most people watching tonight want to know what we've done and what we will do."
The two also differed sharply — once more — over Clinton's vote as a senator in 2002, when Sanders was a member of the House, to support the war in Iraq.
"Experience is not the only point," Sanders said, going at one of the chief rationales for Clinton's candidacy. "Judgment is. And once again, back in 2002 when we both looked at the same evidence about the wisdom of the war in Iraq, one of us voted the right way, and one of us didn't."
"We did differ," Clinton responded, then spun the issue forward by referring to the terrorist group Islamic State, also known as ISIS, and questioning Sanders' capabilities on foreign policy and defense issues. "A vote in 2002," she said, "is not a plan to defeat ISIS."
The issue of Clinton's private email server was raised by Chuck Todd, a co-moderator, who asked whether Clinton could reassure Democrats that a pending FBI investigation would not pose problems should she win the nomination.
She cited reports Thursday that former Secretary of State Colin Powell and others in President George W. Bush's administration had received email containing classified information on their personal accounts. "I never sent or received any classified material," she said. "They are retroactively classifying it. I agree completely with Secretary Powell, who said today this is an absurdity."
Sanders stood by his refusal to attack Clinton on the issue, as Republicans have done with gusto. "I will not politicize it," he said.
Absent from the stage was former Maryland Gov. Martin O'Malley, who quit the race after his poor performance in Iowa.
While still considered the front-runner nationally, Clinton is a decided underdog in New Hampshire, trailing by double digits in recent polls. Part of that is doubtless familiarity with Sanders; New Hampshire voters have a tradition of affinity for their New England neighbors.
Part of it may also be the desire for a contest — the state cherishes its influence in the nomination process, and voters here are notoriously late deciders.
Clinton has been in this position before. She arrived in New Hampshire in 2008 trailing in polls after finishing third in Iowa behind then-Sen. Barack Obama, who hoped to swiftly wrap up the nomination with a primary win. Instead, Clinton's come-from-behind victory set off a nominating fight that stretched all the way to June.
This time she's the one hoping to shut the race down quickly.
But many Democrats, while hoping for a speedy, relatively amicable resolution, are bracing for a rough contest that could last a good while.
Barabak reported from Manchester, N.H., and Halper from Durham. Times staff writers Michael Finnegan in Manchester and Kurtis Lee in Los Angeles contributed to this report.