Joe Biden’s resurrection as a viable candidate has added new tension to the Democratic primary, and has thinned down the field of candidates, with both Pete Buttigieg and Amy Klobuchar dropping out of the race to endorse him. With Elizabeth Warren and Michael Bloomberg yet to catch up in the polls, Biden is now the main challenger to putative frontrunner Bernie Sanders.

But while Biden and Sanders are currently seen as the only two likely nominees, the odds that neither will win the nomination outright are apparently going up — raising the prospect that the nomination will have to be decided at the Democratic National Convention in July.

A “brokered” or “contested” convention is not something the Democrats want. It could deeply divide the party, with supporters of whichever candidate was denied the nomination potentially coming out convinced they were robbed of it by the party elite.

Equally, it’s possible the unedifying spectacle of candidates trading horses and shouting each other down at the last stage of the process could leave whoever is nominated badly tarnished heading into the election.

Nonetheless, with things as they are, it seems the Democrats are unlikely to go to the convention with the nominee already ordained by primary voters.

Super Tuesday: Lively scenes as results flood in across 14 states Show all 29 1 /29 Super Tuesday: Lively scenes as results flood in across 14 states Super Tuesday: Lively scenes as results flood in across 14 states An anti-dairy protester is led away after storming the stage at Joe Biden's Super Tuesday event in Los Angeles, California EPA Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sanders cheer as results are announced at a Super Tuesday party in Texas AFP/Getty Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Joe Biden hold up an election pooster from Barack Obama's 2008 run at a Super Tuesday event for the Democratic contender Reuters Super Tuesday: Lively scenes as results flood in across 14 states Bernie Sander takes to the stage before supporters during his Super Tuesday event in Vermont EPA Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sanders cheer as results are announced at a Super Tuesday party in Texas AFP/Getty Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sanders cheer as results are announced at a watch party in Texas AFP/Getty Super Tuesday: Lively scenes as results flood in across 14 states Joe Biden gestures to suppporters at a Super Tuesday event for the Democratic contender AP Super Tuesday: Lively scenes as results flood in across 14 states Elizabeth Warren waves to supporters at a rally in Michigan as results come in following Super Tuesday Getty Super Tuesday: Lively scenes as results flood in across 14 states A spray painted mural of presidential candidate Senator Bernie Sanders on the side of a building in Kirby, Vernont EPA Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sandes cheer at a Super Tuesday rally in Vermont Getty Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Elizabeth Warren hold sings and cheer in Cambridge Getty Super Tuesday: Lively scenes as results flood in across 14 states Democratic presidential candidate Elizabeth Warren leaves the voting booth at the Graham & Parks School in Cambridge, Massachusetts Getty Super Tuesday: Lively scenes as results flood in across 14 states A poster for sale at a rally for Bernie Sanders on Super Tuesday in Vermont Getty Super Tuesday: Lively scenes as results flood in across 14 states Democratic voter Elliot Zaagman wears a protective mask as he poses for a photo after casting his ballot in Bangkok, Thailand EPA Super Tuesday: Lively scenes as results flood in across 14 states Daisy, a Golden Retriever, outside a polling in San Diego. 1,357 Democratic delegates are at stake as voters cast their ballots in 14 states and American Samoa on what is known as Super Tuesday Getty Super Tuesday: Lively scenes as results flood in across 14 states Democratic presidential candidate Bernie Sanders (greets fellow voters after casting his ballot in his state's primary election at the Robert Miller Community Center in Burlington, Vermont Getty Super Tuesday: Lively scenes as results flood in across 14 states A first time voter stands behind a voting booth in a polling location for the North Carolina primary Reuters Super Tuesday: Lively scenes as results flood in across 14 states Democratic presidential candidate Michael Bloomberg takes part in his Super Tuesday night rally in West Palm Beach, Florida Reuters Super Tuesday: Lively scenes as results flood in across 14 states Fourteen states are holding their primaries with more than one third of the total pledged delegates in the Democratic primaries to be awarded on Super Tuesday Getty Images Super Tuesday: Lively scenes as results flood in across 14 states Presidential candidate Elizabeth Warren and her husband Bruce Mann greet supporters as they walk to a polling site to vote on Super Tuesday in Cambridge, Massachusett EPA Super Tuesday: Lively scenes as results flood in across 14 states Voters cast their ballots at a polling location inside an elementary school on Super Tuesday in Minneapolis, Minnesota EPA Super Tuesday: Lively scenes as results flood in across 14 states Democrat Jamie Wilson displays a sticker after voting in the Super Tuesday primary at John H. Reagan Elementary School in the Oak Cliff section of Dallas, Tuesday, March 3, 2020. (AP Photo/LM Otero) LM Otero AP Super Tuesday: Lively scenes as results flood in across 14 states Democratic voter Le'ana Freeman poses for a photo after casting her ballot at a polling station in Bangkok EPA Super Tuesday: Lively scenes as results flood in across 14 states Voters arrive to cast their ballots at a polling location inside Hunter House at Nottoway Park in Vienna EPA Super Tuesday: Lively scenes as results flood in across 14 states Democrat Jamie Wilson gets a sticker after voting in the Oak Cliff section of Dallas AP Super Tuesday: Lively scenes as results flood in across 14 states A Polling Place sign in the border town of Hidalgo, Texas EPA Super Tuesday: Lively scenes as results flood in across 14 states A girl carries her mothers ballot to the table where she will fill in her choice at the Taylor Elementary School polling location in Arlington Getty Images Super Tuesday: Lively scenes as results flood in across 14 states A man wearing an 'I Voted' sticker and a 'Bernie Abroad' badge after voting in the American presidential primary in Oxford, England Getty Images Super Tuesday: Lively scenes as results flood in across 14 states Stickers for people who vote AFP via Getty Images

Why might a brokered convention be on the cards?

To formally become the Democratic nominee, a candidate must win a majority of delegates to the convention. There are 3,979 “pledged” delegates available this year, meaning a candidate needs to pick up 1,990 to win on the first ballot. These delegates are allocated based on candidates’ performance in the primaries and caucuses held across the states.

While some are allocated on the basis of statewide victories, many are allocated proportionally based on results at lower levels, meaning that even candidates who win relatively few states (or even none) can pick up enough delegates to keep others from winning the nomination outright.

The delegates vote for the nominee in what’s called a roll call vote, where the states’ delegations take turns to announce their votes. If no candidate reaches 1,991 delegates on the first ballot, a second ballot is held.

According to Democratic Party rules, the second ballot includes votes from the party’s 771 “superdelegates” — and that’s where the controversy really begins.

What say do the establishment get?

The superdelegates are a collection of party officials, officeholders and grandees with voting rights at the convention. Under current party rules, they are allowed to vote for whichever candidate they like, but only when the process goes to a second ballot.

They previously had more power, but it was diminished by a rules change after the 2016 primary, in which many Sanders supporters accused the party establishment of stitching up the nomination for Hillary Clinton by declaring their support for her early on.

However, as Elizabeth Warren has pointed out, when Sanders began to trail Clinton in pledged delegates, he himself suggested that superdelegates could hand him the nomination even though he had fallen behind, citing “momentum” in some of the late-voting states.

Sanders has this year changed tack again. He now suggesting the superdelegates should automatically support whoever wins a plurality of pledged delegates, saying that to do otherwise would be to rig the primary against the voters’ choice. Biden has dismissed the idea as hypocrisy: “I wonder where that view was when he was challenging Hillary after she went in with a commanding lead”, he said.

That said, Sanders may have genuine cause for concern. The New York Times has reported that the party establishment is so convinced Sanders would lose to Trump in November that some superdelegates are already hatching plans to prevent him from being nominated.

How likely is this to happen?

A lot depends on how long the remaining candidates stay in the race, and how many delegates they win — as well as whether they release those delegates and/or instruct them to vote for another candidate.

If Warren and Bloomberg fail to win enough delegates to make much of a difference and either Biden or Sanders somehow pulls far ahead, that could defuse some of the tension.

But what’s also worth remembering is that even though talk of a brokered convention for one party or the other surfaces at some point in almost every presidential cycle, the closest any recent convention has come to genuinely “contested” is “fractious”.

In 2008, the Democrats headed into their convention exhausted from a months-long battle between Barack Obama and Hillary Clinton, by the end of which Obama had eked out a narrow pledged delegate lead. While Clinton endorsed Obama just days after the last primaries, some of her more vocal fans were determined to try and peel the nomination away from him.

But in the end, their efforts came to nothing. Clinton nominated Obama in the roll call vote on the convention floor, dispelling any lingering worries of a full-on floor fight.

In 2016, meanwhile, both sides went into their conventions with different problems. The Republicans were set to nominate Donald Trump, whose unstoppable rise had left much of the party establishment aghast; the Democrats, meanwhile, had seen Clinton defeat Bernie Sanders by millions of votes, but not before he roused an anti-establishment movement whose support bordered on devotion.