Soon after Bernie Sanders and Elizabeth Warren effectively, if not quite literally, called each other liars, several shaken progressive activist groups published “A Progressive Unity Statement on the Democratic Presidential Primary.” The statement urged the two candidates, “as well as their campaigns and supporters,” to “find ways to cooperate,” and prevent “an establishment or corporate Democrat” from becoming the party’s nominee.

Since that statement, we have not seen much cooperation. Die-hard supporters of both continue to lob accusations of deceit surrounding what was said by Sanders about whether a woman could beat Donald Trump in 2020. On Saturday, an attendee of a Warren town hall in Iowa disparaged Sanders and said to Warren, “I know what men say to us in rooms, and then what they say to us in person to gaslight us, and I just want you to know that I believe you 100%.” Over in South Carolina, at an NAACP event the same day, Sanders campaign co-chair Nina Turner criticized Democrats who “vote to give [Trump] more money in a military budget”—an implicit swipe at Warren, who voted for the 2017 defense authorization bill.

While there are left-wing populist Democrats who still like both Sanders and Warren, there doesn’t appear to be enough of them to seal the rift that has opened.

Why is the Progressive Unity Statement failing to bring about unity?

There are two obvious reasons. One, the gender issues raised by the conflicting accounts are taken seriously by many Democrats, and cannot be easily swept under the rug. Two, despite the wishes of certain activists, Sanders and Warren are competitors for a single prize, not a mixed doubles tennis team, and have little incentive to work together.

But there’s a third reason, a reason that threatens the entire progressive populist project of uniting to assert control over the Democratic Party:

Unity requires compromise, and the left faction of the Democratic Party has been campaigning against compromise.

The Progressive Unity Statement itself captures the inherent contradictions. The signatories ask Sanders, Warren and their Democratic supporters to unite so they can wage battle against other Democrats. They even explicitly encourage the eventual delegates of Sanders and Warren to prepare for a convention floor flight over the nomination (in their sanitized language, “a unified convention strategy”). They seek progressive unity in order to sow party disunity.

Granted, the signatories stop short of urging abandonment of the Democratic Party should they fail at the convention. “While we firmly believe that either Warren or Sanders should lead our nation in 2021,” they assure, “we will in the end go all-out to defeat Trump no matter who the Democratic nominee is.” But they must know, after the difficulties of 2016, that a rambunctious convention can greatly complicate a unified general election strategy.

These progressive activists deeply disagree with the vision of governing via bipartisan compromise offered primarily by Joe Biden, but also at times by Pete Buttigieg and Amy Klobuchar. They have long expressed frustration with Barack Obama’s insistence on reaching across the aisle, despite the Republican Party’s ruthless legislative tactics.

In turn, Sanders and Warren routinely signal to their supporters that bipartisan compromise should not and need not be eagerly pursued. Sanders promises to muscle his agenda through Congress through “revolution” — meaning a mobilized grassroots that would pressure reluctant members of Congress. Warren pledges to “dream big” and “fight hard,” which includes eliminating the Senate filibuster and packing the Supreme Court to avoid the need for concessions.

So, after demanding an uncompromising approach to politics, progressive activist leaders now want to encourage their rank-and-file to find “solidarity” with those whom they disagree on key issues? Why should democratic socialists find common ground with capitalistic progressive populists? Why should those adamant about breaking the White House glass ceiling step aside for a man who, at minimum, expressed worry about how a woman would fare against Trump?

Of course, those preaching progressive unity would argue that the differences between Sanders and Warren, and between democratic socialists and progressive populists, are small compared to their differences with the more moderate elements of the party. With just some minor compromises among progressives, they would set the stage for, in Warren’s words, “big, structural change.”

The problem is once you tell your people that you can get everything you want if you dream big enough and fight hard enough and refuse to compromise long enough, well, then that’s what your people are going to do.

Solidarity requires focusing on where factions agree, downplaying where they disagree, and accepting that you don’t always get everything that you want. Solidarity, in short, requires compromise.

Once all factions in the Democratic Party accept that fact, they will have an easier time winning elections, and governing after winning.