“If Sanders soars through the first four primaries and Biden and [Pete Buttigieg] stumble, Mike may end up as the only thing standing between Bernie and the nomination.” That’s how Bradley Tusk, who managed Bloomberg’s 2009 mayoral campaign and is advising the presidential run, put it to me on Thursday. Sanders might run away with it, but “a large portion of the party believes that Bernie can’t beat Trump—and that beating Trump is all that matters.”

And so the prospect of a contested convention—in which the Democrats don’t have a presumptive nominee by the time they gather in Milwaukee in mid-July—is more likely than ever.

Read: A short history of Mike Bloomberg supposedly running for president

Even before the past few weeks of surging polls, Sanders aides were counting on winning about 1,500 delegates, short of the 1,990 they’d need to secure the nomination. The primary contest is an often-arcane state-by-state, district-by-district process, a system that makes even the Electoral College seem streamlined. Bloomberg is mounting a hard run at delegates himself. The chances that any candidate clears that 1,990 threshold gets slimmer every week the race goes on.

And here’s where political math and Democratic National Committee rules get important. If a nominee isn’t picked on the first ballot, then superdelegates—about 750 elected officials, party elders, and activists—get to vote in a second round. Sanders aides believe that their only real chance of making him the nominee is winning the first ballot. They’ve started using the phrase significant plurality, and arguing that the superdelegates should give the nomination to whoever’s in the lead, but they’re nervous that this won’t happen. So although they’re looking to crush Senator Elizabeth Warren of Massachusetts, they’re still hoping she’ll stay in the race long enough to win some delegates of her own, then not officially drop out so she can make a deal with Sanders and instruct her delegates to vote for him. (Candidates can keep hold of delegates only if they officially remain in the race.)

Larry Cohen, a Sanders ally who runs a group called Our Revolution, has been quietly talking about a fusion plan for months, keeping in touch with the Working Families Party, which has backed Warren. This effort helped lead to a joint statement from a collection of major progressive groups, including Our Revolution and Working Families, pleading for solidarity for the sake of “a unified convention strategy.” Conversations about that strategy are not happening between the campaigns directly, though Sanders aides do like to chatter about the speculation. Warren’s plan for winning the nomination is to head into the convention with a delegate lead herself.

Biden, for his part, is predicting a long campaign of racking up delegates, though few believe this will be easy if he suffers a series of early losses. Frustration with Bloomberg, whose entire campaign is predicated on Biden’s supposed weakness, is intensifying in the former vice president’s campaign. "The answer to one Republican New York billionaire is surely not going to be a slightly richer Republican New York billionaire,” one Biden ally told me over the weekend. “It's laughable we even have to say that out loud.” Other Democrats are also openly uncomfortable with Bloomberg’s candidacy. After a campaign event in Iowa City on Saturday afternoon, Warren called Bloomberg’s self-funding, skipping-early-states approach “dangerous for our democracy.”