(CNN) Bernie Sanders is facing an uphill battle to win Elizabeth Warren's endorsement, as the Massachusetts senator continues her deliberations just days away from a potentially decisive round of primaries .

After a disappointing Super Tuesday, Sanders is in increasingly desperate need of a lift, as Biden continues to accumulate support not only from moderates, but more liberal lawmakers like Arizona Rep. Ruben Gallego , who endorsed him on Friday after initially backing Sen. Kamala Harris. A Warren endorsement would not guarantee that her base of support shifts to Sanders , but it could provide a shot of adrenaline to a campaign stuck on the wrong side of Biden's momentum.

The Sanders and Warren campaigns have declined to comment on the substance of two conversations -- one on Wednesday and another on Thursday -- between the senators this week. But Sanders campaign manager Faiz Shakir acknowledged his team is seeking Warren's support.

"It could be significant," Shakir told CNN. "There were many states where her impact on the race was crucial to the outcome. If she supported our campaign it has the potential to tip the balance in our favor."

Still, he was resigned to the fact that, after a long and occasionally fractious campaign, Sanders' and Warren's decades-old alliance was in doubt -- and that the time to repair it was running out.

"We've kept the relationship open and cordial from a staff all the way up to the principal level," Shakir said. "We respect her role in this process and believe that her vision for the country best aligns with Sen. Sanders, but the decision is up to her."

Political bond in doubt

But a year of occasionally pointed and personally fraught debates between the campaigns and the candidates has thrown the strength of Warren and Sanders' political bond into doubt. In January, after Sanders denied a report -- and Warren's confirmation of it -- that he told her, in a private 2018 conversation about the upcoming presidential election , that he didn't believe a woman could win, the tensions boiled over in public.

At the end of a Democratic debate in Iowa, Warren refused to shake Sanders' hand and, the frustration evident in her voice, accused him of calling her "a liar on national television."

Some Sanders' supporters online accused Warren of playing to the cameras in order to keep the spotlight on the spat -- a suggestion that still riles former members of Warren's team.

"The people who think that that was intentional on our part -- there's no way," one former Warren aide said. "She was genuinely offended. Bernie could have de-escalated and said, 'I'm sorry if she thought that's what I said. I didn't.' But he didn't even take a half step."

Some of those raw feelings were apparent during Warren's Thursday night interview with MSNBC's Rachel Maddow. During their extended conversation, Warren offered little in the way of hope for Sanders' supporters as she gave her take on both the Vermont senator and Biden.

Warren noted that she and Biden "were in the bankruptcy wars against each other," referencing their famous Capitol Hill clash over a 2005 bill that she vigorously opposed, but then pivoted to talk about her work with him during his time as vice president.

"He's a decent guy," Warren said of Biden.

"And it comes through in pretty much everything he does," she added.

Asked about some Sanders supporters' attacks on her earlier this year, Warren broadened the discussion to include similar incidents -- including an eruption targeting the Working Families Party, after it endorsed her last year, and Nevada's Culinary Union, which was the subject of an angry online backlash after it put out fliers before the caucuses critical of "Medicare for All."

Though she and Sanders had spoken about the issue, Warren told Maddow, their conversation was "short." Pressed on whether he should be accountable for his supporters' behavior, Warren hesitated.

"You know, I shouldn't speak for him," she said. "It's something he should speak for himself on."

Sanders has condemned reports of harassment carried out by his backers, saying, "Anybody making personal attacks against anybody else in my name is not part of our movement." He has also noted that Sanders' campaign officials have themselves been the subject of online abuse. But the tone on social media remains, in many cases, toxic.

What happened in 2016

Despite their political affinity and, at the time, better personal relationship, Warren did not endorse Sanders in 2016. The Massachusetts senator, who was still in her first term at the time, had been the subject of a draft campaign that she ultimately turned down -- a fateful decision that Sanders said led him to launch his presidential primary campaign against Hillary Clinton.

Warren did not pick a side during that heated contest, only announcing that she would support Clinton in June 2016, after the contest was effectively over. With Sanders and Biden now in a one-on-one race that could turn nasty, Warren, who had pitched herself at points as a unity candidate over the last few months, might yet have a chance to do some peace-making -- across the Democratic Party, but also among progressives.

If Warren were to endorse the former vice president, it would be a crushing blow to Sanders, but it could potentially guarantee her -- and progressives -- a position of influence in or around a Biden White House. Staying out of the contest for now could also leave the door open for Warren to play a key role in his administration.

For leaders on the left, the prospect of Warren either swinging to Biden or remaining on the sidelines as the primary hits another crucial stretch has been a source of growing concern.

Ana Maria Archila, the co-executive director of the Center for Popular Democracy, which endorsed Sanders through its political arm last December but remains close with Warren, contemplated the fallout if Warren chooses not to endorse Sanders.

"It would be a sign of real deterioration, of a real fracturing of the movement. And I hope that's not the case," Archila said. "At the staff and base level, the people who endorsed Bernie and the people who endorsed Warren -- we see each other as very much a part of the same movement and aligned around the same purpose.

Shakir said he believed that, in the end, those relationships lead to a political reconciliation.

"Even when we had disagreements and when our policy views differed, at the end of the day we always figured that both sides knew that we would eventually have to come together," he told CNN.

But with each passing day, that prospect now seems increasingly unlikely.