At Tuesday night’s Democratic debate in Des Moines, all eyes will be on Sens. Bernie Sanders (I-VT) and Elizabeth Warren (D-MA) as their once seemingly warm relationship has quickly turned icy.

The Massachusetts senator on Monday night confirmed news reports that her progressive rival told her at a private meeting over a year ago that he did not believe a woman could win in 2020, a development that threatens to upend the nominating contest just weeks before voting commences.

In a statement released by the Warren campaign, the senator responded by saying: “Among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win; he disagreed,” Warren wrote, in part. “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.”

Warren was addressing a CNN story that, according to four sources who spoke to the outlet, Sanders told Warren during a meeting in December 2018 that he did not believe a woman could win the presidency, a claim Sanders vehemently denied, saying that it was “ludicrous” and going on on to accuse “staff who weren’t in the room” of “lying about what happened.”

With recent polling suggesting a closer-than-ever race for a first place finish on the Feb. 3 Iowa caucuses, the two candidates remain solidly in a top tier that features mere single digit differences between them and two other leading Democrats. They will remain in equally close proximity on stage.

But a series of recent events, raising questions about electability and gender dynamics in the presidential race — a topic that plagued Hillary Clinton’s 2016 presidential campaign against Donald Trump — may have rendered the friendly non-aggression agreement between the progressive duo null.

And to be sure, the elbow-throwing starting under the radar well before this week’s debate kickoff.

In early November, a person who appeared to be conducting training calls for Sanders’ campaign instructed another individual to tell prospective voters in New Hampshire, incorrectly, that Sanders was the only candidate running for the Democratic nomination who supports Medicare for All, according to an audio recording obtained exclusively by The Daily Beast.

The call was inadvertently left as a voicemail on a New Hampshire phone number. The first-in-the-nation primary, which votes just one week after the Iowa caucus on Feb. 11, is considered a must-win for both senators.

In the nearly minute-and-a-half-long recording, the trainer explicitly says that Sanders is the sole candidate fighting for that policy, giving the volunteer leeway to tweak the wording as needed.

“What we normally tell people to do is just have a quick version of why you’re supporting him,” the instructor starts off, explaining the script to the second individual. “If I would get to the point in this conversation, I would say: ‘I’m supporting Bernie because he’s the only person fighting for Medicare for All and my family has a lot of medical debt,’” the trainer says.

Sanders and Warren both support a version of the progressive policy, with Warren previously saying “I’m with Bernie on Medicare for All.” In recent months, she faced scrutiny for failing to address specific funding details early on. Sanders, in contrast, has made it the most prominent issue of both his presidential bids.

As the instructor encourages the second individual to elaborate on why she supports Sanders, the individual interjects, “Elizabeth Warren, isn’t she Medicare for All?”

“Yeah she’s Medicare for All, but she’s not advocating for the cancelation of all medical debt, which Bernie is, which is, what I think would be very, very cool,” the trainer responds.

The Sanders campaign did not respond to a request for comment.

The conversation took place over three months ago, on Nov. 6, according to the obtained recording. While the call shows policy contrasts over Sanders’ marquee issue, the previously unreported talking points show a broader effort from Team Sanders to create distance from rivals, including Warren, with whom he shares parts of a broader progressive agenda. Later that month, Sanders’ campaign manager Faiz Shakir told CNN that the Vermont senator is the “last man left on the island” among the pool of Democrats fighting for the nomination on the Medicare for All issue.

In September, Sanders released a proposal that would eliminate $81 billion in past-due medical debt held by 79 million Americans, creating a system where the federal government would pay off unpaid medical expenses that have already gone to debt collectors. Warren has not gone as far in her own plan, and faced a forceful attack from former South Bend Mayor Pete Buttigieg and Sen. Amy Klobuchar (D-MN) on the funding of her proposal during a prior debate.

Now, the revelation that Sanders’ campaign has privately painted him as the only contender who supports that issue could carry new significance as tensions have risen in recent days between the two New England senators, who have previously gone through pains to avoid criticizing the other. At one of the earliest debates, the pair even appeared to team up against more moderate rivals to defend the progressive overhauls their campaigns had proposed.

But as the contests in the early states have tightened, particularly in Iowa and New Hampshire, Warren and Sanders against the world has quickly transitioned into Warren versus Sanders — and it’s getting ugly. Quickly.

Politico reported over the weekend that Sanders’ campaign instructed volunteers to note that Warren appealed to “highly-educated, more affluent people” and say that she was “bringing no new bases into the Democratic Party.”

Warren responded to that report during a campaign stop in Iowa. “We all saw the impact of the factionalism in 2016, and we can’t have a repeat of that,” she said, in an apparent nod to his primary challenge against Hillary Clinton during the last presidential election. “Democrats need to unite our party.”

“I hope Bernie reconsiders and turns his campaign in a different direction,” she said.

Adam Green, co-founder of the Progressive Change Campaign Committee, which has endorsed Warren, said he hopes the attack doesn’t spill over to Tuesday night.

“Most voters don't care to see Bernie's attack on Warren carry onto the debate stage,” Green told The Daily Beast in a statement. “But his attack underscores Warren's closing argument — that her inspiring message plus years of building coalitions instead of dividing people gives her a unique ability to unify Democratic voters and win swing voters to defeat Trump.”

At least one notable figure, the president of the United States, is rooting for a slugfest.

“Bernie Sander’s volunteers are trashing Elizabeth “Pocahontus” Warren. Everybody knows her campaign is dead and want her potential voters,” Mini Mike B is also trying, but getting tiny crowds which are all leaving fast. Elizabeth is very angry at Bernie. Do I see a feud brewing?” President Donald Trump wrote on Twitter on Monday.

The Daily Beast previously reported that senior members of Trump’s re-election campaign and well-placed Republican officials have at various points of the election cycle expressed increased anxiety over both Warren and Sanders. When Warren was surging in momentum during much of the summer, some in Trump’s inner circle privately mused that nothing seemed to stick to her, and that, to their dismay, she showed resilience against broader GOP attacks.

Others in Trump’s team have long been reluctant to write Sanders off, particularly following the endorsement from the influential freshman Rep. Alexandria Ocasio Cortez (D-NY) in October, privately saying he’s likely to be a formidable contender in the election.

The president, known for obsessing about polls, has seen both Warren and Sanders both surge at different points in the primary campaign. A new Des Moines Register survey released late last week showed Sanders with a narrow edge, coming in at 20 percent, just 3 points ahead of Warren’s 17 percent, the second highest. A separate poll from Monmouth University released on Monday shows a murkier outcome: former Vice President Joe Biden leads by 24 percent, followed by Sanders at 18 percent, Buttigieg at 17 and Warren in last among the top four at 15 percent.

“Even among the top contenders, about 4 in 10 supporters are still open to changing their initial preference by caucus night. There are only three weeks to go, but this race is far from over,” Patrick Murray, director of the Monmouth University Polling Institute, said.