In the 2016 Republican presidential primary, Donald Trump was greatly helped by two factors: a deeply splintered opposition, and the largely winner-take-all rules of GOP primaries. Voters who opposed Trump never coalesced behind another candidate and eventually he was able to win the nomination with less than 45 percent of the total votes cast.

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.

But it's not so simple. The Democratic primary in 2020 will work quite differently from the Republican one four years ago. The party's primary process doles out delegates on a proportional basis and the crowded field means it will be quite possible for nobody to win on the first vote at the 2020 convention. 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.

To see why, let's review the basic structure of the Democratic primary, which turns out to be monumentally complicated.

Delegates are awarded to states according to a formula based on each one's share of the total Democratic vote for president over the last three elections, plus their Electoral College votes. U.S. territories and colonies get some delegates as well. This makes for a rough base delegate count of 3,278. Unlike the GOP race in 2016, these "pledged" delegates will be awarded on a proportional basis. Every candidate who can muster 15 percent support will get at least something in each primary or caucus (subject to some additional rules accounting for the fact that sometimes delegate counts do not divide up so evenly), and these delegates must vote for the candidate to whom they were awarded in the first ballot at the convention.

Then there are additional "unpledged" delegates (or "superdelegates") rounding out the total delegate count, consisting of members of the Democratic National Committee (DNC), Democratic members of Congress, and other party bigwigs. At last count, there were 766 superdelegates. But per new rules instituted since 2016 (more on that later), these delegates only get to vote if the convention goes to a second ballot — or if one candidate has locked up so many pledged delegates it would make no difference.

But wait, there's more! The DNC requires that all primaries or caucuses are to take place between March 3 and June 9. Any state that disobeys will get a 50-percent penalty on their delegate total — except for Iowa, New Hampshire, Nevada, and South Carolina, who get specific exemptions to run their caucuses or primaries earlier.

On the other hand, the DNC also gives states bonus delegates for delaying their primary. States that hold their primary or caucus in April get an additional 10 percent, while ones that do so between May 1 and June 16 get a 20 percent bonus. Then states that cluster together in groups of three or more starting March 24 will get another 15 percent. (So for instance if three neighboring states buddy up for an April 15 primary, they will all get 25 percent extra delegates.)

The objective here is presumably to incentivize states away from stacking up at the very beginning to maximize their leverage, and also to stick together so it's easier for candidates to campaign in one regional bloc. But one side effect is that it is still unknown what the exact delegate total is, because some states have not finalized their primary date yet. New York, for instance, only recently locked in its primary for April 28.

And that brings me finally to the dread contested convention. If nobody has enough pledged delegates to win on the first ballot, then the superdelegates will get to meaningfully vote, and the pledged ones will be released from their commitment. At this point, delegates will be able to vote for anyone who meets the eligibility requirements, whether or not they actually won any primaries.

And as New York's Ed Kilgore argues, while a multi-ballot contested convention has been the perennial long-shot obsession of pundits for generations (and one hasn't happened since 1952), it's genuinely possible this time. Biden, Elizabeth Warren, and Bernie Sanders all appear to have solid bases of support putting them over the 15 percent threshold in many states, and tons of cash. Meanwhile Cory Booker, Pete Buttigieg, and Kamala Harris are reportedly also doing quite well money-wise, and can thus continue to bleed off support and make it harder for anyone else to assemble a commanding lead.

This brings me back to Biden versus Sanders and Warren. 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. (A Sanders or Warren nomination put over the finish line by superdelegates would at least be very funny.)

It is faintly ridiculous that considerations like this could end up deciding who is the next president. If the Democratic Party wants regional blocs whose primaries don't all come at the same time, they could just do that — and perhaps with some ranked-choice voting on top to allow for instant runoffs in case nobody can assemble a majority. There is far too much money and energy being spent in the Democratic primary on tactical considerations aside from who can get the most support.

But the rules are the rules, and there is no changing them at this point. Things may change in future, but at this point Sanders and Warren are better for the left separate than together.

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