DES MOINES — After nearly an hour, the question that progressives had been fearing finally arrived. Senator Bernie Sanders laughed. Senator Elizabeth Warren did not.
“I didn’t say it,” Mr. Sanders insisted, with Ms. Warren turning his way, as he denied her explosive account that he told her privately in 2018 that a woman could not win the presidency.
“Bernie is my friend,” Ms. Warren replied firmly, disputing his memory, “and I am not here to try and fight with Bernie. But, look, this question about whether or not a woman can be president has been raised and it’s time for us to attack it head-on.”
All through this Democratic primary, voters have worried aloud about the thorny subject of electability, wondering if a woman — even a woman they might support — would be able to defeat President Trump.
In her exchange with Mr. Sanders on Tuesday, Ms. Warren hoped to turn the issue on its head, noting that of all those onstage, only the women, Ms. Warren and Senator Amy Klobuchar of Minnesota, had won all of their major elections and later observing that the party’s success in the 2018 midterms was powered largely by female candidates and voters.
She also made it known that no one else here had defeated a Republican incumbent in the last three decades, a statistic Mr. Sanders moved to rebut, citing his House victory in 1990.
“Wasn’t that 30 years ago?” she asked, turning to the crowd like an actor breaking the fourth wall in a sitcom.
“I beat an incumbent Republican congressman,” Mr. Sanders repeated, emphasizing that 1990 was indeed 30 years ago.
“I don’t know if that’s the major issue of the day,” he concluded.
But the context was.
In seeking to defuse any concerns about a potential female nominee, Ms. Warren appeared to see no option but to extend, if not expand, a feud her advisers claim she never wanted.
Since the beginning of the primary campaign, Mr. Sanders and Ms. Warren have proceeded with a patina of comity, focusing on their mostly shared policy goals and reminding anyone who would listen about their ostensibly genuine mutual admiration.
“Bernie and I have been friends for a long, long time,” Ms. Warren said last month in Ottumwa.
“Elizabeth Warren is a very good friend of mine,” Mr. Sanders told reporters last weekend in Iowa City.
But competitive campaigns tend to test the definition of the word.
Perhaps this moment, or something like it, was always going to come — the natural consequence of a contest with these stakes, of two candidates fighting for so many of the same voters.
It was easy enough to project friendship and allegiance all last year as dual progressive dreamers, tag-teaming to make the case against the more incremental politics of a front-runner like former Vice President Joseph R. Biden Jr. It is quite another thing to see the current state of the primary — with wide-open races in Iowa and New Hampshire and, in theory, room enough for only one liberal standard-bearer as the calendar turns — and maintain a fully united front.
Sunday was fraught: Ms. Warren said she was “disappointed” in Mr. Sanders amid reports that his campaign had distributed a script to volunteers instructing them to depict Ms. Warren as out of touch.
Monday was worse: CNN reported that Mr. Sanders told Ms. Warren during a private meeting in 2018 that a woman could not win the presidency. Mr. Sanders forcefully denied having made the remarks. Ms. Warren said that Mr. Sanders had in fact raised doubts about a woman’s electoral viability.
“I thought a woman could win; he disagreed,” she said in a statement on Monday. “I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry.”
Entering Tuesday’s debate, progressive groups had spent the preceding 48 hours in something approaching full-scale panic, alarmed that a skirmish between two largely like-minded candidates would serve only to benefit more moderate alternatives like Mr. Biden and former Mayor Pete Buttigieg of South Bend, Ind.
The question of whether a woman can defeat Mr. Trump has been the long-whispered soundtrack of much of this Democratic primary, invoked constantly in voter interviews among even supporters of candidates like Ms. Warren and Ms. Klobuchar.
Ms. Warren had not directly addressed gender as forcefully as she did Tuesday night, when she vowed, in her closing statement, to become “the first woman president of the United States of America.”
She has more frequently talked about Aunt Bea — a wonder woman in her life — who made an appearance in an exchange about child care.
It was a story she has told many times before, but the circumstances on Tuesday made it newly resonant.
“If I hadn’t been saved by my Aunt Bea, I was ready to quit my job,” Ms. Warren said. “And I think about how many women of my generation just got knocked off the track and never got back on.”
As late as last week, it seemed as if the debate was shaping up primarily as a clash between Mr. Sanders and Mr. Biden. Since an American airstrike killed Iran’s top military commander, the two men have been sparring over foreign policy. Mr. Biden has seized on the escalating tension to highlight his experience while Mr. Sanders has used it as grounds to promote his longtime focus on international diplomacy. Mr. Sanders, a vocal opponent of the war in Iraq, has also repeatedly and aggressively hit Mr. Biden on his vote to authorize it.
The sparring over foreign policy has delighted Mr. Sanders’s advisers, who have long ached for direct conflict with Mr. Biden: Not only is he a moderate foil to Mr. Sanders’s democratic socialism, but he also in many ways represents the establishment Washington that Mr. Sanders loathes.
Mr. Biden was bracing for the fight. But when it came time for Mr. Sanders to go on offense, he settled occasionally for jokes.
“I would not meet with — absent preconditions — I would not meet with the, quote, ‘supreme leader,’ who said ‘Joe Biden is a rabid dog, he should be beaten to death with a stick,” Mr. Biden said about the leader of North Korea.
Mr. Sanders butted in: “Other than that, you like him?”
“Other than that, I like him,” Mr. Biden confirmed.
As ever on Tuesday, Mr. Biden presented himself as the candidate Mr. Trump fears most. “I’ve been the object of his affection now more than anybody else in this stage,” he said.
While Mr. Biden is not an enviable debater on his best day, he seemed to largely survive the evening without a significant misstep — no small thing as he continues to lead most national polls and edges into contention in surveys of Iowa and New Hampshire, where he has long struggled.
That no candidate has yet emerged as a decisive front-runner in Iowa has made voters’ decisions here all the more complicated as they strain to identify someone who can defeat Mr. Trump. Only 40 percent of likely Democratic caucusgoers have made up their minds, according to a Des Moines Register poll released last week; nearly half said they could be persuaded to support another candidate, and 13 percent said they did not have a first choice.
Of course, the locals have also found excitement in the stress, making the proceedings on Tuesday the hottest ticket in a cold town. Leaving a restaurant on Sunday, several Iowans asked a Buttigieg campaign official if he could help them get into the event hall. The official demurred.
The debate on Tuesday went forward against the relentless din of Washington news, from the Iran affair to a looming Senate impeachment trial that could sideline half of the candidates onstage. (The forum also excluded the Democrat most ubiquitous in television advertising: Michael R. Bloomberg, the billionaire former New York City mayor who is not competing in the early-voting states.)
The candidates and moderators did not wind toward Mr. Trump’s congressional fate until nearly the end. There was talk of the Constitution. There was talk about duty. “Some things,” Ms. Warren said, “are more important than politics.”
And some seemingly unimportant actions can appear politically meaningful.
At the end of the evening, as the candidates wrapped up more than two hours of televised talking with several minutes of televised farewells, Mr. Sanders extended his hand to Ms. Warren. She did not reciprocate, beginning a brief conversation that ended without a handshake. Mr. Sanders raised two open palms — as if to say: enough of this — and walked off.
Sydney Ember reported from Des Moines, and Matt Flegenheimer from New York.