DES MOINES — The two leading liberal presidential candidates battled furiously Monday on the fraught topic of gender in American politics as they prepared to meet in Iowa for a debate that may be the last high-profile gathering before the state's caucuses.

The question of whether a woman could be elected president in the United States was fueled by a CNN report that cited sources saying Sen. Bernie Sanders (I-Vt.) had told Sen. Elizabeth Warren (D-Mass.) that a woman could not win the White House in 2020.

Sanders heatedly denied the report, which his campaign manager Faiz Shakir told CNN was “a lie.”

“It is ludicrous to believe that at the same meeting where Elizabeth Warren told me she was going to run for president, I would tell her that a woman couldn’t win,” Sanders said in a statement.

Two people with knowledge of the conversation at the 2018 dinner at Warren’s home told The Washington Post that Warren brought up the issue by asking Sanders whether he believed a woman could win. One of the people with knowledge of the conversation said Sanders did not say a woman couldn’t win but rather that Trump would use nefarious tactics against the Democratic nominee.

“What I did say that night was that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could,” Sanders said in the statement. “Do I believe a woman can win in 2020? Of course! After all Hillary Clinton beat Donald Trump by 3 million votes in 2016.”

Warren’s campaign issued a statement late Monday saying that the fate of a female candidate was among the topics “that came up.”

With the Iowa caucuses and New Hampshire primary approaching, an international crisis gave the race for the democratic presidential nomination a new urgency. (The Washington Post)

“I thought a woman could win; he disagreed,” she said, adding: “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.”

The dispute played out in advance of a Des Moines debate whose importance is heightened by an unprecedented circumstance: Soon after the debate, three of the six participants — Sanders, Warren and Sen. Amy Klobuchar (D-Minn.) — will depart for Washington as impeachment trial of President Trump begins.

The unpredictable impeachment schedule means the three probably will have only a limited amount of time in Iowa in the three weeks before the Feb. 3 caucuses, curtailing their ability to connect with voters here in the final stretch of the campaign.

It was not immediately clear how the gender dispute would be taken by those voters. Since Hillary Clinton’s 2016 loss to Donald Trump, many voters have expressed a deep concern about the odds of another woman defeating him for reelection. Former vice president Joe Biden touched on the topic earlier this month when he noted that Clinton faced “unfair” sexism during her campaign.

“That’s not going to happen with me,” Biden said.

Warren increasingly has worked to revive the anti-Trump spirit of the 2017 Women’s March and persuade women to side with her.

A September Washington Post poll found 23 percent of Democrats and Democratic-leaning independents said a man would be more likely than a woman to defeat Trump in November.

The broader question of electability has hung over the campaign since its inception. Going into Tuesday’s debate, four of the top contenders have sharpened their arguments about why they represent the strongest challenger to Trump in November.

Biden is expected to highlight his foreign policy experience, according to an adviser who spoke on the condition of anonymity to discuss debate deliberations. The topic has risen in the minds of many voters amid recent skirmishes with Iran, and Biden has offered himself as a safe bet against a president many in the party loathe.

The senior adviser said Biden will try on Tuesday to make the case that his eight years of experience in the White House standing next to the commander in chief make him the right choice.

“I think he will say Trump’s dangerous actions speak to why it’s so important to hire someone who, on day one, can start to pick up the pieces,” the Biden adviser said. “Biden has that experience. He’s sat in the Situation Room. He has extensive experience on the Foreign Relations Committee.”

Biden, whose camp had long downplayed his chances in Iowa, has ramped up his schedule in Iowa, recently surpassing Warren for the number of events held in the state. He has more than doubled his spending on television ads — with his campaign and a super PAC backing him spending more than anyone else over the past two weeks. He has landed the state’s most coveted endorsements, sent out-of-state surrogates in to campaign on his behalf and is planning significant time in the state over the next week.

“We have long had a theory of the case in Iowa and elsewhere is when push comes to shove, caucusgoers fundamentally care about one thing above all else: that’s beating Trump,” said Pete Kavanaugh, Biden’s deputy campaign manager.

Sanders heads into the debate homing in on Biden’s foreign policy record as well, but to make the opposite point: Biden has a history of making the wrong call at key moments, Sanders has argued, including his vote to authorize war with Iraq. On the electability front, Sanders has argued that his top fundraising numbers, swelling crowd sizes and lead in some polls show he can excite the party and win.

While it was unclear how Sanders would approach the flap with Warren at the debate, his campaign advisers have signaled in recent days that they are much more interested in drawing a contrast with Biden, particularly pressing on his past comments about cutting Social Security and building on an argument pushed out by a top Sanders surrogate over the weekend that Biden has as spotty record on supporting African Americans.

Warren plans to paint herself as a unity candidate who would be acceptable to both the liberal and establishment wings of the Democratic Party. The leak of Sanders’s alleged comments also could play into her new emphasis on gender in recent weeks.

Pete Buttigieg, the former mayor of South Bend, Ind., is hoping to stay above the fray and show he can attract voters who left the party to support Trump.

A spokesman for the Buttigieg campaign declined to comment on debate strategy, but the candidate has been drawing a few key points of distinction between him and his fellow candidates over the past few weeks, including shots at Biden’s vote in favor of the Iraq War.

Only six candidates will be on the debate stage, the smallest group so far to participate. It also will be the first debate featuring only white candidates, a lack of diversity underscored Monday when Sen. Cory Booker (D-N.J.), who is African American, dropped out of the race citing as one reason his inability to qualify for the debate.

Four candidates — Sanders, Biden, Warren and Buttigieg — are bunched at the top of the polls leading into the caucuses. Yet, with three weeks to go, the race remains fluid — 45 percent of Iowans said they could be persuaded to back a different candidate in a Des Moines Register/CNN poll last week.

“It’s not unexpected to have Iowans taking their time considering all the candidates. But when you have people saying they’ve narrowed it down to their top four or top three choices, we’re in new territory,” said Matt Paul, who was Clinton’s Iowa state director in 2016. “There’s going to be such high interest in this debate.”

The effort to persuade prompted disputes between the candidates over the weekend. Sanders’s team tangled with Biden’s team over Biden’s vote for the war in Iraq. Nina Turner, a national co-chair of the Sanders campaign, wrote an op-ed published Sunday in a South Carolina newspaper that Biden “has repeatedly betrayed black voters to side with Republican lawmakers and undermine our progress.”

But the fight between Sanders and Warren, who previously had criticized each other only in passing, eclipsed that feud.

Sanders said it was “sad that, three weeks before the Iowa caucus and a year after that private conversation, staff who weren’t in the room are lying about what happened.”

Warren informed Sanders at the meeting that she planned to run for president, according to one of the people with knowledge of the conversation.

The fight followed a weekend quarrel over a Politico report that the Sanders campaign issued a script painting Warren as an elitist for volunteers calling voters.

Warren said she was “disappointed” and warned of a repeat of what she referred to as the “factionalism” that cost Democrats the White House in 2020.

Adam Green, a Warren ally, said Warren plans to make a broader closing argument on the big themes of her campaign and focus the energizing nature of her campaign.

“Warren’s closing argument is that she can best unite and energize all factions of Democrats, plus win swing voters in the general,” said Green. “This debate is all about big-picture vision and electability against Trump.”

Matt Viser, Cleve R. Wootson Jr. and Chelsea Janes contributed to this report.