Since declaring his candidacy for president on April 25, former Vice President Joe Biden has taken a commanding lead in the race for the Democratic nomination.

Indeed, despite much carping from critics about how he was a paper tiger whose polling would crumple the moment it came into contact with reality, Biden's announcement bump has been sustained and substantial. On the eve of his entrance into the race, he led Bernie Sanders by an average of just over 6 points, with Kamala Harris, Elizabeth Warren, Pete Buttigieg and Beto O'Rourke bunched together in the mid-to-upper single digits. But while Sanders' numbers have eroded along with the others, Biden has surged, opening up nearly a 20-point gap in polls.

The only candidate to have gained traction since Biden's entry is Warren, whose relentless barrage of hotter-than-a-graphite-fire policy proposals may finally be breaking through with voters. She was even nipping at Bernie's heels in the latest Quinnipiac poll. The upshot is that we are now much closer to the basic moderate-progressive schism that many predicted and that we've seen in previous cycles. The trouble for progressives is that, even though their wing of the party might be ascendant, Sanders and Warren have the potential to divide their vote all the way to Milwaukee, allowing the more center-left Biden to claim the nomination. If they want to stop him, they're going to have to talk one of their preferred candidates into dropping out of the race.

This is not surprising. Warren and Sanders are very similar figures and surely each was secretly hoping that the other would decide not to run. According to the DW-NOMINATE vote-scoring system, a measure developed by political scientists to describe politicians' ideology, Warren has been the most progressive senator in the Democratic caucus since she took office in 2013. In fact, the system actually ranks her as the most left-wing senator since Oregon's Wayne Lyman Morse, who died in 1974, and places her to the left of anyone from the New Deal era. Sanders is close behind, ranking fourth in the same time period. Both base their populist appeal on the idea that the struggles of ordinary Americans can be traced to the outsized power and influence of wealthy elites.

The difference is in their style. “America's middle class is under attack,” Warren declared in her cinematic announcement video. “Billionaires and big corporations decided they wanted more of the pie, and they enlisted politicians to cut 'em a fatter slice.” Her solution is a dizzying combination of entitlement expansion and frontal assaults on the richest Americans, including a wealth tax, student debt forgiveness, universal childcare, anti-trust action and more. The specificity and often shocking aggressiveness of these policy proposals is central to her appeal, but she also uses the soothing and familiar political language of markets and “the middle class” to avoid the perception that she would precipitate some kind of rupture with normal politics.

For Sanders, the policy details are often secondary to the task of building a sustainable movement. He rarely talks about the middle class, preferring instead to lump the destitute, the working poor and the middle classes together and pit them against the “1 percent.” In his announcement, he noted, “If we are prepared to stand together, there is no end to what the great people of our nation can accomplish.” It is bottom-up energy, his “political revolution,” that will force a reckoning with inequality, racism and climate change.

These two visions are not necessarily mutually exclusive, and their policy prescriptions are often similar, with the significant exception of health care, an animating issue for Sanders but something that seems much less central to Warren's story of what's wrong with America. Warren sees the system of for-profit insurance and thinks it can be reformed, incentivized and and regulated into compliance with basic human decency. While she pays homage to Medicare-for-all as a long-term goal, she believes it must be approached in stages, and that in the interim more needs to be done to rectify the abuses and injustices of the status quo. Sanders, on the other hand, has been uncompromising about the need to immediately seek a radical break.

Either Warren and Sanders, if elected, would be by far the most progressive president in American history, and would represent a sharp and undeniable shift from the post-Reagan economic policies of the Democratic Party.

The victory of a longtime establishment figure like Biden, however, would be a bitter disappointment to energized progressives, particularly younger voters, hoping to bring sweeping change to American public policy. And it's becoming ever more obvious that the only way for either Warren or Sanders to get a clean shot at taking Biden head-on is for one of them to get out of the way.

To be clear, Biden's lead is so big right now that even just adding the vote shares of Sanders and Warren together wouldn't catch him. And given what we know about second-choice data for each candidate, Biden would capture a significant share of either candidate's core supporters. The most recent Morning Consult poll suggests Biden would get 37 percent of Sanders voters and 20 percent of Warren voters if they dropped out, a reminder that there is no such thing as a “lane” in presidential primaries and voters consider much more than policy preferences when choosing candidates.

That said, Sanders is the clear second choice of Warren voters, and Warren is right behind Biden for Sanders supporters. The departure of one from the race would give a substantial boost to the other. And it certainly feels like they are getting in each other's way. Sanders shaved a few points off of Warren's early February polling when he announced, and her recent (modest) spike in support also looks like it is coming partly at his expense. Neither commands an army of mindless sycophants eager to do what their god says, but it is also true that dropping out and endorsing the other could convince most supporters to switch loyalties fairly seamlessly — especially if, in doing so, a good case is made for why the nomination shouldn't go to Biden.

Of course, having invested time, money and soul into the effort to win the nomination, neither Warren nor Sanders will be dropping out anytime soon, especially not with hours of free media coming to them as part of the endless debate process, and with the Iowa caucuses still more than eight tedious months away. Nor should they. Both candidates and their supporters deserve a long chance to take the case directly to the primary electorate, to build momentum and to cut into Biden's lead.

It is also possible that the primary becomes a three-way race after Super Tuesday, and that Biden pulls into the convention short of a pledged delegate majority, while Warren and Sanders combined have him beat. While that would give progressives a chance to claim the nomination, it also relies on the nightmare scenario of taking it away from the delegate leader, which would be both unlikely and unwise.

A more likely scenario is this: at some point, whether in September, in January or in March, either Warren or Sanders will have to fall on the sword of progressivism in an effort to help the other get elected. If neither is willing to do so, they'll both be left fighting for primetime speaking slots in Milwaukee while Biden preps his acceptance speech.