Bernie’s heart condition aside, his momentum in the 2020 Democratic primary has slowed, even begun to backslide compared to his earlier support in the Spring. With Sen. Elizabeth Warren on the rise, and former vice president Joe Biden still holding around 26% of the electorate, is it time for Bernie to bid adios to the 2020 primary and let his backers coalesce around the other progressive stalwart in the race?

For Bernie fans, that question might sound insulting, but looking at how Donald Trump won the Republican Primary may be instructive if Democrats want to avoid nominating Joe Biden by default.

Ryan Cooper at The Week tackled this question in September and came to the conclusion that Biden is leading the Democratic primary much like Trump led the Republican primary:

Thus far, Joe Biden is in a similar position to 2016 Trump in terms of polls — considerably ahead of a divided field, but well short of an absolute majority. So it raises a question: If progressive Democrats want to prevent a Biden nomination, with Elizabeth Warren and Bernie Sanders nearly tied behind him, how best might they act together to keep him from winning? Splinter’s Hamilton Nolan argues that one or the other should drop out to clear the way for the other, while the Working Families Party’s national director Maurice Mitchell justified his organization’s endorsement of Warren this week by arguing she could better beat Biden. “If our focus is on victory, we can’t be delusional about it,” he told The New York Times.

It’s very true that the Working Families Party, which endorsed Bernie in 2016, bypassed him in 2020 in favor of Warren. That decision was not without controversy in WFP ranks, but it was a signal that some of Bernie’s 2016 backers don’t see his chances as realistic in 2020 and instead want to compromise enough to win. WFP’s endorsement decision was driven by their belief that Warren – not Sanders – is best-positioned to defeat Joe Biden in the Democratic primary.

There is, however, one vast difference between Trump’s win of the 2016 Republican Primary and it comes down to the way the two parties award their delegates. Republicans use “winner take all” rules which means as Trump won primary races, even by just a couple of percentage points, he would take all the delegates from that state.

In contrast, Democrats hand out delegates proportionally to candidates. Usually, the threshold requires receiving at least 15% of the vote, but it means that in a tight primary race where the top 3 candidates are just a few points apart, they all will be awarded some delegates.

That proportional delegate dynamic, argues Cooper, means that there is less reason for Bernie, or Warren, for that matter, to consider dropping out early:

So long as Warren and Sanders are pulling from different demographic categories, and both can remain above the 15 percent threshold for getting some delegates, it makes sense for both of them to stay in and try to beat Biden at the convention. Indeed, either one dropping out at this point might well help Biden win.

It wouldn’t seem that Bernie supporters would flock to Joe Biden over Elizabeth Warren and add to his support. It would be more likely to see some of Warren’s backers go mostly to Bernie, but a sizable part may flock to Biden. However, according to the data, it’s surprising how things might shake out depending on which candidate dropped out and whether you believe the polling to be accurate:

The key question for whether either one should drop out is where their supporters will go. If all Sanders voters will flow to Warren if he leaves, and vice versa, then it might make sense to consolidate the left vote under one banner — but polling shows this to be not true. Their supporters are quite different demographically, with Sanders’ more diverse and less educated, and Warren’s whiter, richer, and more educated. The top second choice for Sanders supporters is actually Biden (26 percent versus 24 percent for Warren). Meanwhile, Sanders is the top second choice for Warren supporters, but only by a small margin (24 percent for Sanders versus 21 percent for Biden). Voters often have weird preferences like that.

Now, polling is not dispositive here; supporters might be swayed by endorsements and such. But the evidence we do have suggests that either one dropping out would boost Biden’s polls by a potentially decisive margin. So long as both Warren and Sanders are behind Biden but above the 15 percent threshold, it may make good tactical sense for both to stay in the race, and to try to win by combining forces at the convention — at which point superdelegates might actually decide the outcome for once, despite the post-2016 reforms reducing their influence.

Cooper’s point is that even if Bernie wanted to help vanquish Biden in the primary by dropping his candidacy and pushing supporters to Warren, he might not be doing her a favor. There is a sizable number of his supporters that would still end up backing Biden instead, which might push him back into the mid-30s of the support in the field.

Biden’s numbers have softened even since Cooper’s article came out back in mid-September. At the moment, according to the RCP average, Biden and Warren are only separated by less than 3% support. If Bernie’s 16% support ended up splitting 60/40 in favor of Warren, which is probably a high estimate for Biden, it would probably put Warren in the lead by a few percentage points.

At the moment, Bernie has no inclination to leave the race despite his recent stent procedure and recovery keeping him off the campaign trail. The assumption is that he’ll be back out in a couple of weeks, and his major money haul will ensure that he can keep going well into next year. He has the resources to keep fighting, but the question will come eventually whether he decides that he can’t risk the chance of helping Biden win with a small plurality of Democratic voters.

Then again, what if he does decide to pull the plug on Bernie 2020 and endorse Warren as the successor to his democratic socialist brand? We’d see a new race overnight between Warren and Biden duking it out head-to-head as a near certainty.