Bernie Sanders was pointing — to the left, to the right, at himself, at the sky — in one moment coming under such ubiquitous duress that he extended both hands outward, as if pushing to the front of a crowd, to swat back stage-wide accusations of fuzzy math.
His rivals said he was the preferred choice of Vladimir Putin against President Trump. They linked him indirectly to the Charleston church massacre of 2015, citing his past opposition to gun control measures. They suggested he would be a uniquely weak candidate in the fall. This was the first 11 minutes.
“I’m hearing my name mentioned a little bit tonight,” Mr. Sanders said playfully.
He could get used to that.
Onstage for the first time as the unambiguous front-runner and object of his peers’ attention, Mr. Sanders made clear in the debate in Charleston, S.C., on Tuesday that the changing circumstances — strong showings in South Carolina and on Super Tuesday could propel him to a runaway delegate lead — would not much change the man.
He was never surprised, never entirely smooth and, when it was done, not necessarily looking any less the favorite than he did going in.
Yet as Mr. Sanders moves to expand and consolidate his hold on the Democratic primary, he at once reinforced the reservations that many in the party still have about him and laid bare the power and peril of his politics: his own unyielding worldview — and just how unlikely he is to adapt it to anyone else’s definition of electability.
In one heated exchange about his history of sympathy for socialist and communist governments, he said that he “opposed authoritarianism all over the world” but that there were times when certain regimes might still deserve praise.
“When dictatorships, whether it is the Chinese or the Cubans, do something good, you acknowledge that,” he said. “But you don’t have to trade love letters with them.”
It was an arresting response sure to invite more criticism from his competitors, and also one strictly in line with the fervently anti-imperialist ideology that has long guided him.
With more than 40 percent of pledged delegates in the primary at stake between Tuesday night and the next scheduled debate in mid-March, the candidates seemed aware, at last, of the urgent electoral hour.
But after other campaigns had long insisted that Mr. Sanders could be stopped when the time came — done in by his own stridency, surely; or his heart attack; or the murky cost estimates for his plans — Tuesday night supplied striking evidence that such predictions were something closer to wishful thinking.
The debate reinforced the difficulty of sticking labels on someone who wears them all conspicuously anyway: Caricatured as an incorrigible lefty, Mr. Sanders treats “democratic socialist” not as an attack but as an accurate description of his vision. Called a reckless spender, he apologizes to no one for the scale of his platform.
Most slights on Tuesday were familiar, if punchier than usual.
Former Vice President Joseph R. Biden Jr. said Mr. Sanders had contributed to “carnage on our streets,” alluding to the senator’s voting record on gun legislation. (Mr. Sanders said he had cast “thousands of votes, including bad votes,” adding that Mr. Biden had cast a bad vote of his own to authorize the war in Iraq.)
Pete Buttigieg, the former mayor of South Bend, Ind., hit Mr. Sanders for refusing to support abolishing the filibuster. “How are we going to deliver a revolution if you won’t even support a rule change?” he wondered.
Senator Elizabeth Warren had perhaps the most complicated argument to make, saying that Mr. Sanders was winning “because the Democratic Party is a progressive party” but comparing her own record of accomplishment favorably to his. “Progressives have got one shot,” she said, “and we need to spend it with a leader who will get something done.”
No candidate entered the evening with more repair work to do than Michael R. Bloomberg, the billionaire former New York mayor whose jarring stumbles during last week’s debate had shaken confidence in his smartest-guy-in-the-room pitch to Democratic voters. While he had some steadier moments on Tuesday, presenting himself as more electable than Mr. Sanders at every opportunity and talking up his philanthropic history with gun control, Mr. Bloomberg slogged through some lowlights once more.
Challenged again by Ms. Warren over nondisclosure agreements with women at his company amid allegations of workplace mistreatment, Mr. Bloomberg had trouble landing an apology for making “jokes” that employees did not appreciate. “I don’t remember what they were,” he said. “If it bothered them, I was wrong, and I apologize.”
Later, he had to catch himself, while discussing his spending during the 2018 midterms, from beginning to say he had “bought” the House majority for Democrats: “All of the new Democrats that came in, put Nancy Pelosi in charge and gave the Congress the ability to control this president, I bought — I, I got them.”
At one point, in a discussion of redlining, Mr. Bloomberg stopped — “since I have the floor for a second” — to reflect on the humbling of the past week before “my fellow contestants.” “I’m surprised they show up,” he said, testing a little self-deprecation, “because I would have thought after I did such a good job in beating them last week, that they would be a little bit afraid to do that.”
The debate gathered the contenders in a state with a large black Democratic electorate, after a month that began with two nominating contests in predominantly white states. But the setting only reinforced how far the 2020 primary had traveled in recent months. Here was a stage full of white people — four septuagenarians, two billionaires, one septuagenarian billionaire — left to represent a field that once looked more like the country, with candidates of color and several more women onstage for much of the past year.
Mr. Biden, who counts black voters as the core of his political base, is relying on a victory in the state to lift him into Super Tuesday next week. Other more moderate options, like Senator Amy Klobuchar and Mr. Buttigieg have struggled to connect with nonwhite audiences.
Mr. Buttigieg allowed on Tuesday that he was “conscious of the fact that there are seven white people on this stage talking about racial justice.”
If success in Nevada proved that Mr. Sanders could win in a diverse state, the primaries in South Carolina and on Super Tuesday will be the greatest test yet of his capacity to build a big-tent coalition and make good on his longstanding pledge to expand the electorate.
Still, winning is winning, and Mr. Sanders has done the most of it in these early stages of the primary, delivering him to an unusual perch: the clear center of attention.
Aides to Mr. Sanders had been anticipating all manner of criticism — “there are a lot of knives out,” a top adviser, Jeff Weaver, said over the weekend — and the senator had actively prepared for how to deflect them. Since the Nevada caucuses, his rivals have been presenting him as dangerously out of step with the country, suggesting that as the party’s presidential nominee Mr. Sanders would jeopardize Democrats’ chances of keeping the House of Representatives and winning the Senate.
Mr. Sanders has provided some early hints as to how he would respond to certain critiques. When pressed on how he planned to pay for his extensive policy agenda during a CNN forum on Monday, he whipped out a sheet of paper.
“This is a list,” he said triumphantly, “which will be on our website tonight of how we pay for every program that we have developed.” It was a clever stunt, but it also revealed that he may not be as prepared as he wants people to believe: The document was limited, and in some cases, the revenue Mr. Sanders earmarked did not actually add up to the cost. Nothing he said Tuesday offered further clarification.
But the debate did not alter a fundamental truth about Mr. Sanders: In race after race during his decades in politics, he has often proved an elusive target, brushing off political vulnerabilities and shrugging through adversity. Those who know him attribute his durability to his consistency and the loyalty of his base, which is more likely to rally around him after an obvious attack than desert him because of it.
At the evening’s end, the candidates were asked to name a “misconception” about them. Mr. Sanders lit up. Viewers had been hearing it all night, he suggested.
“Misconception,” he said, “is that the ideas I’m talking about are radical.”