The deepening rift among the two progressive candidates threatens to lock the Democratic Party’s left wing out of the White House. With hours to go until roughly a third of delegates are awarded, anxiety among progressive leaders is rising, and Warren, after her weak performance in the first four voting states, is under growing pressure to drop out from some Sanders supporters but notably not the Sanders campaign itself.

“If a candidate pulls ahead after [Tuesday], we need to put aside our differences and unite on behalf of economic and racial justice,” Aimee Allison, founder of the organization She the People, said of Warren and Sanders. “The country is ready for new ideas, and we have two leading progressive campaigns that have so much to offer millions of Democrats. The moderates are playing to win. It’s time we rose up as a movement to play to win as well.”

Democratic presidential candidate Sen. Bernie Sanders. | Andy Clayton-King/AP Photo

But the split may be difficult to mend. Warren allies and some aides have long felt Sanders could have done more to rein in his surrogates, staffers and supporters from attacking Warren — and they were upset when grassroots Sanders backers hissed at the mention of her name and tweeted snake emojis at her.

Some Sanders aides and allies, meanwhile, feel as if Warren's team is too sensitive to criticism from some campaign aides and is wrongly holding them responsible for tweets from people outside their operation. Others also still suspect she or people close to her planted a January CNN story about him allegedly telling her a woman couldn't beat President Donald Trump — as a way to kneecap him. Sanders denied ever saying that.

Alexandra Rojas, executive director of the left-wing group Justice Democrats, called on Warren to stop “attacking” Sanders and pledge to "give her delegates to him if he has more votes to ensure a progressive wins the nomination.” Justice Democrats has not endorsed in the race.

Evan Weber, national political director of the pro-Sanders Sunrise Movement, likewise said Warren and Sanders “should publicly commit to grouping their delegates together at the convention in order to ensure that a progressive can win against the establishment” if they both stay in the race after Super Tuesday.

The Warren campaign did not respond to those statements.

There was one ray of hope this week for progressives fearful of a divided front, particularly those who favor the Vermont senator: Democracy for America and The Nation both endorsed him Monday. The publication encouraged Sanders to "do more to discourage his supporters from engaging in personal attacks,” while pressing Warren to recognize "criticism by her of Sanders or his record only benefits their common enemies."

But some progressives remain split, even if they don't want to further the disconnect. Though for months liberal groups and officials have been coalescing behind Sanders, leading figures on the left have failed to consolidate in the past few days anywhere close to the degree moderate elected officials have. Despite finishing behind Buttigieg, who suspended his campaign Sunday and then endorsed Biden, Warren has shown no signs of exiting the race. Instead, she has promised to fight all the way to the convention.

Some progressive activists are debating what to do about the predicament. One of the largest immigrants rights groups, United We Dream, issued a co-endorsement of Sanders and Warren this week.

It’s almost a mirror image of the place moderates found themselves in weeks ago, when centrist presidential candidates tore into each other in debates and attack ads while Sanders racked up victories. At the time, moderate insiders complained that they lacked a coordinated strategy to unite against Sanders and were mostly talking instead of doing.

"There's a lot of, 'Oh, my god, what's going on?' networked chatter from person to person that I imagine will turn into deeper conversations over coming days,” said a person involved in efforts to unify progressives. Tuesday “could make things more clear or more confusing."

Some of Sanders’ supporters in left-wing media have been calling on Warren to drop out for the good of the progressive cause.

And some Warren supporters argue there will be a backlash to pushing out the last viable woman in the race. “I think the media and other politicians are underestimating the unmitigated fury a lot of women — like me — will feel if Warren is forced out because of a massive pressure campaign to intimidate the millions of voters who actually prefer her," said Liat Olenick, a public school teacher and progressive activist in Brooklyn who supports Warren. "We need women to turn out in November."

The Working Families Party and Progressive Change Campaign Committee, which support Warren, have argued that it is beneficial for Sanders to have her in the race because she attracts voters who would switch to Biden if she dropped out, a point even some Sanders supporters concede.

Other Sanders backers believe that analysis doesn’t take into account the boost he would receive if she simultaneously left the race and endorsed Sanders. A Morning Consult national poll found that Biden’s support jumped by 10 percentage points after Buttigieg and Klobuchar exited the race and threw their weight behind him, putting him in the lead with 36 percent, followed by Sanders with 28 percent.

Faiz Shakir, Sanders’ campaign manager, said his team has made calls to progressive groups and officials in recent days to build support as moderates consolidate. “For anyone who’s wondering whether we are making efforts to try to expand our tent, yes. We are doing so,” he said.

The prospect of Warren and Sanders and their supporters coming together could be made more difficult by the realities of the rocky primary, however. After attempting to stick to a nonaggression pact throughout most of the campaign, the relationship between the two camps has become increasingly fraught. Some Warren surrogates and aides — long frustrated by what they considered cheap shots from Sanders’ team and supporters — are increasingly hitting back.

When Matt Bruenig, a prominent Sanders backer and founder of the left-wing think tank People's Policy Project, said that “most of [Warren’s] career was corporate bankruptcy,” longtime Warren policy aide Bharat Ramamurti snapped back: “This is an outright lie, as are many of the things you say about her and her ideas.” Ramamurti quickly deleted the tweet, but it was further evidence of frayed nerves.

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Sanders aides and surrogates, too, have been aggravated by Warren’s recent attacks on Sanders as well as her campaign’s declaration that she will go all the way to the convention as her campaign predicts no candidate will have the required delegate majority.

“If your progressive candidate is running to be Joe Biden’s vice president then maybe it’s time for a rethink. Like, before Tuesday,” tweeted Sanders surrogate and professor Naomi Klein. (She later told POLITICO, “I do not tweet as a surrogate.")

But with Biden surging, the two progressives could have a common enemy to unite around. On Monday, Warren signaled that she is turning her attention to Biden, telling a crowd in Los Angeles, "No matter how many Washington insiders tell you to support [Biden], nominating their fellow Washington insider will not meet this moment. … Nominating someone who wants to restore the world before Donald Trump, when the status quo has been leaving more and more people behind for decades, is a big risk for our party and our country.”

Sanders, too, declined repeatedly to take the bait this week at a news conference when asked about Warren’s role in race and whether her super PAC is funded by special interests.

Sara Nelson, president of the Association of Flight Attendants, predicted a shift in strategy in upcoming days.

“The issues that Bernie Sanders and Elizabeth Warren are talking about are the issues that are energizing the Democratic base,” she said, adding that after Super Tuesday, “I think that a lot of people in the progressive movement will gladly talk about how we coalesce around the issues that we care about.”

