DES MOINES — The Democratic presidential candidates clashed in starkly personal terms Tuesday over who had the best chance to defeat President Trump, as Senator Elizabeth Warren sought to jump-start her campaign in the last debate before the Iowa caucuses by highlighting her electoral success and that of other female candidates in the Trump era.
Prompted by the moderators, Ms. Warren and Senator Bernie Sanders continued a back-and-forth over the fraught subject of whether a woman could be elected president, an issue that in recent days had caused the first serious breach in their relationship. One day after she confirmed a report that Mr. Sanders had told her in a private meeting that he did not think a woman could defeat Mr. Trump, Ms. Warren trumpeted her Senate victory over an incumbent Republican and then gestured down the debate stage toward the four male candidates.
“Collectively they have lost 10 elections,” she said, before acknowledging the only other female candidate present, Senator Amy Klobuchar. “The only people on this stage who have won every single election they have been in are the women: Amy and me. And the only person on this stage who has beaten an incumbent Republican in the past 30 years is me.”
Mr. Sanders, Ms. Warren’s top rival for progressive support, flatly denied that he had made the comment when the two lawmakers met without aides in 2018. He said it was “incomprehensible that I would think that a woman couldn’t be president of the United States,” noting Hillary Clinton had won the popular vote in the 2016 general election.
Meeting less than three weeks before Iowans vote, the Democrats disagreed over international affairs and keeping troops in the Middle East, whether to support Mr. Trump’s trade deal for North America, how aggressively to tackle climate change and, yet again, health care. But the issue animating much of the evening was the same question that has shaped the primary race for the past year: which of them would be the most formidable contender against Mr. Trump.
The contest has increasingly revolved around questions of electability, but the matter has become more urgent in the weeks since hostilities increased between the United States and Tehran after the killing of Maj. Gen. Qassim Suleimani, the powerful Iranian commander. Much of Tuesday’s debate, which featured six of the remaining candidates, touched on national security as the Democrats excoriated Mr. Trump, urged caution in the Middle East and laid claim to the mantle of being the best potential commander in chief.
Former Vice President Joseph R. Biden Jr. came under far less scrutiny than his standing as the national front-runner might have merited in the final debate before voting begins in Iowa on Feb. 3. Just as notable, Pete Buttigieg, the former mayor of South Bend, Ind., who has slipped in Iowa, seemed satisfied to make his own case without sharply criticizing his top rivals.
New polls in Iowa show that Democratic voters are roughly split between four top candidates: Mr. Biden, Mr. Sanders of Vermont, Ms. Warren of Massachusetts and Mr. Buttigieg.
But while Mr. Sanders was criticized for the cost of his plans, Ms. Warren for how many people would be turned off by hers and Mr. Buttigieg for the scope of his ambitions, Mr. Biden went long stretches on Tuesday receiving scant attention.
The debate unfolded at an extraordinarily volatile moment in American politics, with impeachment looming and escalated tensions with Iran. Yet befitting the setting and the stakes of the debate, multiple candidates — Ms. Warren, Mr. Buttigieg, Mr. Sanders and Ms. Klobuchar — were just as eager to invoke Iowa and stories of specific Iowans they had met along the campaign trail, tailoring their pitch to the crucial state.
But the contretemps between Ms. Warren and Mr. Sanders was the most memorable moment in the lead-up to the caucuses here. It was a remarkable exchange between the two senators, in part because they are friends and have labored to abide by a de facto nonaggression pact for the past year. But more important, it also crystallized the competing cases that the leading Democratic contenders were making for why they were best positioned to defeat Mr. Trump.
Even as Ms. Warren said “Bernie is my friend and I am not here to try to fight with Bernie,” she flashed him a smile after Mr. Sanders noted that he, like Ms. Warren, had once defeated an incumbent Republican. “Just to set the record straight, I defeated an incumbent Republican running for Congress,” he said, before Ms. Warren pointed out that it had been 30 years ago.
Acknowledging that she was facing doubts about her chances to defeat Mr. Trump, she pointed out that John F. Kennedy had addressed questions about his Catholicism and, more recently, Barack Obama had overcome doubts that he could win the presidency as a black man.
Both times, Ms. Warren said, “the Democratic Party stepped up and said yes.”
It was an unusual closing argument in Iowa for a candidate who first rose to contention on the basis of her policy proposals, but it reflected the urgency she was facing to reverse her decline in a state where she led in the polls last year.
For his part, Mr. Sanders did not seem rattled by the confrontation, at least during the forum. But in the immediate aftermath of the debate, CNN cameras captured Ms. Warren appearing to refuse to shake Mr. Sanders’s hand, and the two senators engaged in what seemed to be a pointed conversation.
Tom Steyer, the California billionaire, who was standing nearby during the exchange, said later in the spin room, “Whatever they were going on between each other, I was trying to get out of the way as fast as possible.”
During the debate, Mr. Sanders used the exchange with Ms. Warren to make his own case for why he was the most electable candidate: because he could lure a stream of new voters to the polls. “The real question,” he said, “is how do we beat Trump? And the only way we beat Trump is by a campaign of energy and excitement and a campaign that has, by far, the largest voter turnout in the history of this country.”
Mr. Biden, who has placed his own polling strength against Mr. Trump at the center of his candidacy — including in a new television ad timed with Tuesday’s debate — was just as emphatic that he was best equipped to win the general election.
“The real issue is who can bring the whole party together,” said Mr. Biden, citing his endorsements from a variety of Democrats, including many who were racial minorities. “I am the one who has the broadest coalition of anyone running up here.” Later, alluding to Mr. Trump’s attempt to press the Ukrainians to investigate the work of his son in Ukraine, Mr. Biden said of the president: “I’ve taken all the hits he can deliver.”
Ms. Klobuchar cited her success appealing to a range of voters in her home state of Minnesota and even boasted that every one of her Republican opponents had left politics since they lost to her. “I think that sounds pretty good with the president we have right now,” she said.
But Ms. Klobuchar struggled momentarily when she sought to highlight the success of other Midwestern Democratic women and forgot the name of Gov. Laura Kelly of Kansas before receiving a cue.
“Kansas has a woman governor right now and she beat Kris Kobach,” she began. “And her name, um, is, I’m very proud to know her, and her name is, uh, Governor Kelly. Thank you.”
Mr. Steyer, whose television ads have blanketed Iowa and the other early-voting states, said his economic experience would position him well against a businessman turned president. “Whoever is going to beat Donald Trump is going to beat him on the economy,” he said, “and I have the experience and expertise to show he’s a fake there and a fraud.”
Not long after the debate, Mr. Trump responded on Twitter: “Steyer is running low on cash. Nobody knows him. Made his money on coal. So funny!”
In a rare policy split, Mr. Sanders and Ms. Warren disagreed on the new North American trade deal that Mr. Trump is trying to push through Congress. Mr. Sanders said it was not worth supporting — even if it made a “modest” improvement. “We can do much better than a Trump-led trade deal,” he said.
Ms. Warren, however, said that was the reason to support it. “We have farmers here in Iowa who are hurting,” she said.
The back-and-forth was an example of how Ms. Warren has sought to position herself as a progressive more willing to get things done than Mr. Sanders.
The candidates clashed, as they have in all of the debates, on health care. Mr. Sanders was pressed by the moderators about the cost of his “Medicare for all” package; unlike Ms. Warren, Mr. Sanders has not said what his proposal would cost or revealed which taxes he would increase to pay for it.
Ms. Warren, who has found herself on the defensive over her embrace of Medicare for all in recent months, pitched her plan as more expansive than those offered by Ms. Klobuchar or Mr. Buttigieg.
“You would kick 149 million Americans off their health insurance,” Ms. Klobuchar said to Ms. Warren at one point. “It’s just not true that the plan I’m proposing is small,” Mr. Buttigieg said at another, suggesting Ms. Warren’s approach would turn off too many voters.
“We’ve got to move past a Washington mentality that suggests that the bigness of plans only consists of how many trillions of dollars they put through the Treasury, that the boldness of a plan only consists of how many Americans it can alienate,” Mr. Buttigieg said pointedly.
Mr. Buttigieg was asked directly about his lack of support among black voters, whom he will need to activate not just to win the nomination but also a potential general election against Mr. Trump. Mr. Buttigieg said those who know him best — in South Bend — support him, cited his African-American backers in Iowa and noted that his new campaign co-chairman was a member of the Congressional Black Caucus.
Mr. Sanders had the opportunity right from the start to emphasize his pacifist credentials as the debate opened with questions about the heightened tensions with Iran and who was best positioned to serve as commander in chief. Mr. Sanders immediately seized the opportunity to trumpet his past opposition to the war in Iraq. “I not only voted against the war, I helped lead the effort against the war,” he said.
Mr. Sanders warned that both the Iraq and Vietnam wars had been based on “lies.” “Right now, what I fear very much is that we have a president that is lying again and could drag us into a war that is even worse than the war in Iraq,” he said.
Mr. Sanders drew a distinction with Mr. Biden, who had supported the Iraq war resolution in the Senate. “Joe and I listened to what Dick Cheney and George Bush and Rumsfeld had to say,” Mr. Sanders said. “I thought they were lying. Joe saw it differently.”
Mr. Biden said he regretted his vote for that war. “It was a mistake and I acknowledge that,” Mr. Biden said, while noting that as vice president he had brought thousands of troops home from the Middle East.
The race here remains extremely fluid. A large part of the electorate is up for grabs in a contest that many of the campaigns believe will produce record-setting turnout. A Des Moines Register-CNN poll last week indicated that 45 percent of caucusgoers said they could still be persuaded to support a different candidate.
In a final twist that had the potential to affect the race in Iowa, four candidates who will be deliberating on Mr. Trump’s fate are confronting a looming challenge: how to mount a successful Iowa campaign while their duties in Congress require them to be in Washington.
With the senators likely to be in the Capitol for up to six days a week for the impeachment trial of Mr. Trump, they will be unable to make their final appeals to Iowa voters in person as frequently as Mr. Biden and Mr. Buttigieg.
“Some things,” Ms. Warren said, “are more important than politics.”