For a moment — a few even — this one felt different, as circumstances and medical prudence required.
Joe Biden and Bernie Sanders began with an elbow tap. They stood behind lecterns six feet apart, in keeping with federal health guidelines. They rarely interrupted each other, initially, subsisting on mutually nonaggressive meditations on the menace of the coronavirus and how to corral it.
“This is a national crisis,” Mr. Biden said early in Sunday’s debate. “I don’t want to get this into a back and forth in terms of our politics here.”
But that is the trouble with a debate — a back and forth in terms of their politics, by definition — in the age of pandemic. The politics can spread quickly.
In their first one-on-one session of the 2020 primary, destined to be unusual for its context, the candidates set off on an often strikingly punchy evening, pocked with enough voice-raising and purposeful gesticulating that the two-yard rhetorical demilitarized zone was repeatedly breached.
Mr. Sanders hit the former vice president for his record on Social Security, bankruptcy, abortion rights, gay rights. “America, go to the website right now!” the Vermont senator instructed, requesting a third-party ruling during an exchange over Mr. Biden’s past comments about federal benefits. “Go to the YouTube right now.”
Mr. Biden, eager to bring the primary to a functional close after amassing a solid delegate lead, seemed to grow frustrated that his nominal olive branches — gentle praise for, and policy gestures toward, Mr. Sanders’s progressive vision — were being snapped in half.
“He’s making it hard for me right now,” Mr. Biden said, when asked how he might appeal to Sanders supporters. “I was trying to give him credit for some things — he won’t even take the credit.”
Yet if the tone of the proceedings was certain to disappoint some Democrats hoping to bring the party together against President Trump, the debate was also in many ways a venue for Mr. Sanders’s most recognizable role: the crusading underdog, out to unsubtly move the conversation leftward.
His were attacks he has been practicing for days, attacks that his aides have pushed behind the scenes and on Twitter. He unveiled versions of the material at rallies recently in St. Paul, Minn., and in Phoenix and during short made-for-television sessions with reporters — swings he had long been reluctant to take at someone he considers a friend.
“Come on, Joe, you’re an honest guy,” Mr. Sanders said at one point on Sunday night.
“Why don’t you just tell the truth here?” he said at another.
“I will talk to the governor!” he replied in one flourish, after the former vice president suggested his rival consult Gov. Andrew M. Cuomo of New York on Mr. Biden’s history with minimum wage policy.
The two candidates have, at least publicly, processed the virus in ways that reinforce their central theories of this political era. Mr. Biden has held up the slapdash federal response as evidence that Mr. Trump must be driven from office. Mr. Sanders has suggested that the pandemic only heightens the urgency for wide-scale upheaval of the American health care and economic status quo.
“Let’s be honest,” Mr. Sanders said, “and understand that this coronavirus pandemic exposes the incredible weakness and dysfunctionality of our current health care system.”
Mr. Biden pointed to Italy, whose health care system is groaning under the stress of the coronavirus, to argue that Mr. Sanders’s signature policy proposal, Medicare for all, was not the answer.
“With all due respect to Medicare for all, you have a single-payer system in Italy,” he said. “It doesn’t work there.”
It was not clear what the public appetite might be for a debate in these times. Sunday evening was always going to be weird, eerie — a reminder, without a live audience, of how far the nation has traveled in the handful of days since its people were advised not to travel far at all.
One question centered on the candidates’ approaches to crisis hygiene.
“I’m using a lot of soap and hand sanitizers,” Mr. Sanders said.
“I make sure I don’t touch my face and so on,” Mr. Biden reported.
The forum was, if nothing else, a live event to be shared with a large audience. Gone was the viewership competition from N.B.A. games, baseball spring training, a college basketball tournament selection show.
Why not watch two septuagenarians talk for two hours?
Since taking control of the primary with commanding victories over the past two weeks, Mr. Biden has saluted Mr. Sanders’s policy instincts, to a point, mindful that he will need the senator’s supporters to join him in the fall if he is the nominee. (This did not dissuade him from a swaggering aside about his electoral success in the face of Mr. Sanders’s superior resources at the time. “I didn’t have any money, and I still won!” Mr. Biden enthused, as Mr. Sanders dinged him for being supported by the ultrarich.)
In recent days, Mr. Biden has embraced versions of plans put forth by Mr. Sanders and Senator Elizabeth Warren, whose progressive campaign for the presidency ended earlier this month, as the former vice president moves to shore up support on the left.
In one extraordinary turn on Sunday, Mr. Biden committed for the first time to having a woman as his running mate. Mr. Sanders said he would “in all likelihood” but did not go quite as far.
But their pledges underscored another fundamental truth: Only the idea of women — as potential vice presidents, as people in need of legislation to protect their reproductive rights — appeared onstage; for the first time in the 2020 election cycle, no women were actually debating.
Even when the two men were discussing bankruptcy, Mr. Biden only glancingly nodded at Ms. Warren, acknowledging that “she should get credit,” while taking some credit himself for now agreeing with her.
Some of the exchanges risked feeling out of place during a time when a lethal virus is sweeping the globe — two men arguing in a television studio as the world around them cracks.
Near the end, they clashed over their foreign policy views, resurfacing a dispute that seemed potent last month but perhaps less so now.
Asked about his qualified praise for Fidel Castro, Mr. Sanders said he opposed authoritarianism “whether it’s in Cuba, whether it’s Saudi Arabia, whether it’s in China or whether it is in Russia.”
“But,” he added, “to simply say that nothing ever done by any of those administrations had a positive impact on their people, would I think be incorrect.”
Mr. Biden, however, forgetting about unity for a spell, was not content to let the fight go.
“I’m prepared to compare my foreign policy credentials up against my friend here on any day of the week and every day of the week,” he said.
Perhaps it was unrealistic for Biden allies to expect much deference from Mr. Sanders — and for anyone to expect Mr. Biden, rarely cited for his discipline, to play the reserved statesman all night.
During a news conference on Wednesday, a day after Mr. Sanders suffered a second night of big primary losses, he registered an ideological wish list of policy issues as if to offer Mr. Biden a record of concessions he might seek if he were to drop out.
“What are you going to do?” Mr. Sanders asked Mr. Biden again and again from a lectern at a hotel in Vermont, challenging his rival to address the systemic problems he has long placed at the heart of his progressive agenda.
In the earlier days of the contest, such a head-to-head matchup like the one Sunday night would have been a dream scenario for the Sanders campaign. Aides have long viewed Mr. Biden, an establishment moderate, as the perfect foil for Mr. Sanders’s promise of a political revolution. They have often ached for Mr. Sanders to challenge Mr. Biden more directly.
It may now be too late. Though there were previous flashes of confrontation, it was not until recent days that Mr. Sanders began to aim squarely at Mr. Biden’s vulnerabilities.
But if Mr. Sanders will always fight for his agenda, as his long history of doing so has made clear, the debate may be the last time Mr. Sanders faces Mr. Biden as an opponent. Though he has not given any indications he will leave the race — and he and his team say he is still running to win — the upcoming primary calendar, which includes states like Florida and Ohio on Tuesday, does not exactly play to his strengths. The goal may now be as much about accumulating delegates to use as leverage in negotiations with Mr. Biden as it is about winning.
There may also be another benefit to staying in: keeping his people motivated. Some of his supporters are prone to view any Sanders exit as the fault of the establishment — a view Mr. Sanders himself has at times encouraged. Should he leave later and on his terms, his most zealous fans may be less inclined to dismiss the outcome as rigged — and more willing to participate in the election in November.
Or maybe Mr. Biden, still viewed skeptically by much of Mr. Sanders’s young and progressive base, can make the sale himself.
Taking a cue from Mr. Sanders, Mr. Biden tried to highlight his double-digit average contribution (“forty-four dollars”), rejecting the charge that his campaign is “somehow being funded by millionaires.”
More familiar was Mr. Biden’s closing message, summoning his own personal tragedies in a bid to demonstrate the kind of empathy he can no longer showcase in rope lines with voters.
When the debate ended where it started, with a discussion of the coronavirus, Mr. Biden spoke about loss, briefly calling it unimaginable. Then he stopped himself.
“I guess I can imagine,” he said, “the fear and concern people have.”