WESTERVILLE, Ohio — Senator Elizabeth Warren looked down, performatively taken aback. She raised her hand to speak — surely it was her turn again. She shrugged a little.
For about an hour on Tuesday, Ms. Warren had been the prime target of her debate rivals, compelled to defend as never before the hard-charging progressivism and soak-the-rich economic approach that has elevated her to the top of the polls. Beto O’Rourke, the former Texas congressman, had a theory about all of that.
“Sometimes, I think Senator Warren is more focused on being punitive or pitting some part of the country against the other,” he said, using a question about the wealth tax to lash Ms. Warren’s broader political philosophy, “instead of lifting people up and making sure this country comes together.”
Ms. Warren turned to Mr. O’Rourke, then back to the cameras. “So, um, I’m really shocked at the notion that anyone thinks I’m punitive,” she said.
Perhaps. But she should not have been surprised.
For months, Ms. Warren had moved largely unimpeded in her brisk jog to the front of the 2020 Democratic pack, coasting through debates without incident as her calls for “big structural change” took hold and former Vice President Joseph R. Biden Jr. absorbed the unforgiving glare afforded the favorite. Time and again this year, moderators had invited Ms. Warren’s top competitors to attack her. Time and again, they had done so gently, if at all.
This time, Mr. O’Rourke went after her. Pete Buttigieg, the millennial mayor of South Bend, Ind., did the same early in the evening in a slashing exchange on health care. Andrew Yang said she was wrong on the wealth tax. Senator Kamala Harris smiled as she and Ms. Warren sparred over whether to regulate President Trump’s tweets. Mr. Biden initiated his most direct debate-stage confrontation with Ms. Warren to date, saying she was “being vague” in campaign proposals.
This was Ms. Warren’s reward for achieving co-front-runner (and maybe outright front-runner) status: persistent sniping from fellow Democrats who see her surge as the most urgent threat to their own paths to the nomination. Ms. Warren greeted the deluge with mixed success, never wobbling too precariously but retreating at times to the safe harbor of stump-speech platitudes. On occasion, she appeared so eager to avoid the fray that she could give the impression that she was not engaging with the substance. “A yes-or-no question that didn’t get a yes-or-no answer,” Mr. Buttigieg observed at one point.
The fresh antipathy was all the more striking for its contrast with the treatment of two fellow contenders whose campaigns have been consumed by drama of late. Shortly after his recent heart attack, Senator Bernie Sanders attracted little meaningful criticism, on policy matters or his health. He will receive a boost this weekend with the expected endorsement of Alexandria Ocasio-Cortez, the New York congresswoman who has emerged as an impassioned gatekeeper of the left.
And Mr. Biden, straining to keep his grip on the race, survived an early dissection of the impeachment inquiry that centers on Mr. Trump’s urging of the Ukrainian president to investigate Mr. Biden and his son Hunter.
While the candidates plainly saw little incentive in questioning whether Hunter Biden had traded on the family name in dealings abroad — lest they be seen as doing the president’s bidding — their deference itself was damning: Other campaigns have long expected Mr. Biden to falter on his own, viewing Ms. Warren as the more nettlesome long-term headache, and the former vice president’s performance on Tuesday quite likely did little to alter their calculation.
The moment that Mr. Biden had prepared for came early. Asked about his son in the debate’s opening minutes, Mr. Biden worked to summon the righteous fury and stern statesman’s gaze perfected over his half-century in public life.
“My son did nothing wrong,” he said firmly. “I did nothing wrong.”
At times, his delivery was wobbly, as it tends to be. He stopped and started a bit. He cited George Washington. But he worked toward the conclusion he has been repeating often on the campaign trail.
“He doesn’t want me to be the candidate,” Mr. Biden said of the president. “He is going after me because he knows if I get the nomination, I will beat him like a drum.”
Before the debate, several rivals had come to Mr. Biden’s defense, plainly mindful of the limits and potential downsides of condemning peers so far this year — and of condemning Mr. Biden on this subject in particular. None made an issue of Mr. Biden’s family on Tuesday.
At the previous three debates, Mr. Biden had been the focus of attacks both glancing and sharply personal. Yet the candidates who have gone after Mr. Biden frontally — including Ms. Harris; Julián Castro, the former federal housing secretary; and Senator Kirsten Gillibrand of New York, who has left the race — have seen few lasting benefits. In some cases, such as Mr. Castro’s, the effort appeared to backfire with some Democratic voters and officials who are eager to keep the focus on Mr. Trump.
Taking on Ms. Warren brought risks of its own, given her popularity with the party’s base and the scant evidence throughout this primary that voters are inclined to reward infighting of any sort.
But less than four months before the Iowa caucuses, her competitors have determined that complacency will not suffice.
Mr. Buttigieg was the first aggressor, a few minutes into the debate in Westerville. He had been asked about Ms. Warren’s support for Medicare for All and her squishy responses to the question of whether middle-class taxes would rise under it. This was the candidate with “a plan for everything,” Mr. Buttigieg taunted, “except this.”
Ms. Warren’s head shot skyward. “We can pay for this,” she insisted, repeating that “costs” would rise only for the wealthy and declining to concede — as Mr. Sanders, her comrade-in-health-care-policy, has — that middle-class taxes would go up.
Amy Klobuchar, a moderate Senate peer who has leveled few attacks from the stage all year, was having none of it. “At least Bernie’s being honest here,” she said.
The debate, the fourth of the campaign, came during a period of momentum for Ms. Warren, who has moved into a lead position, topping Mr. Biden in some surveys both nationally and in early-voting primary states. At the previous debate, Mr. Biden quickly abandoned mannerly efforts to draw contrasts with her, and other high-polling rivals had until Tuesday largely refrained from issuing piercing criticism onstage.
But in the lead-up to the debate, several contenders had telegraphed arguments against Ms. Warren. At a fund-raiser last week, Mr. Biden made an oblique jab, saying that to claim that Medicare for All is achievable without a significant increase of taxes “not just for the wealthy but across the board is just not honest.” And on Tuesday morning, Mr. Buttigieg released a digital ad that swiped at Ms. Warren and Mr. Sanders over their support for Medicare for All, a sweeping single-payer plan that would all but eliminate private health insurance.
The two leading candidates had avoided flashes of explosive confrontation with each other until around the final half-hour of the debate, when Mr. Biden said, “I’m the only one who has gotten anything really big done,” criticizing Ms. Warren and Mr. Sanders for advocating overly general ideas on issues like health care. Ms. Warren went on to point to her role in helping to create the Consumer Financial Protection Bureau during the Obama administration.
“I agreed with the great job she did,” Mr. Biden said. Turning to face Ms. Warren, jabbing his hand in her direction, the former vice president’s voice rose. “And I went out on the floor and got you votes. I got votes for that bill. I convinced people to vote for it. So let’s get those things straight, too.”
Some in the room applauded.
“I am deeply grateful to President Obama,” she said pointedly — as his vice president grinned — “who fought so hard to make sure that agency was passed into law, and I am deeply grateful to every single person who fought for it and who helped pass it into law.”
“You did a hell of a job at your job,” Mr. Biden said, interrupting her.
Less clear was Mr. Biden’s appraisal of Ms. Warren in her new role: the co-favorite.