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Balloons pour from the ceiling as delegates celebrate after Democratic U.S. presidential nominee Hillary Clinton accepted the nomination on the fourth and final night at the Democratic National Convention in Philadelphia, Pennsylvania, U.S. July 28, 2016. REUTERS/Carlos Barria

What is a contested convention, and why does it matter in 2020?

For former Vice President Joe Biden and Vermont Sen. Bernie, winning the Democratic nomination got a little less complicated on Thursday, when Sen. Elizabeth Warren announced she was dropping out of the 2020 presidential race. But with plenty of primaries left to go, they still face challenging party divisions and new rules governing how the party leadership gets to weigh in on the potential nominees at the convention.

Biden and Sanders are not the only Democrats left standing — Hawaii Rep. Tulsi Gabbard is still in the race after winning two delegates from American Samoa. But the two front-runners zoomed ahead of their rivals on Super Tuesday when Biden picked up 10 wins, and Sanders added to his delegate haul with states like California and Colorado.

Biden’s comeback was a development not many predicted. And in a race full of surprises, political analysts have asked whether 2020 will see a “contested convention,” in which no candidate wins a majority of delegates to the National Democratic Convention in July. That outcome seems less likely now that there are fewer contenders to split the liberal and moderate factions — but it’s not impossible.

Here’s how we got to the current delegate system, and how some of the messier dynamics could all play out later this year.

First, the basics: How does the Democratic delegate process work?

In the presidential election cycle, caucuses and primary contests help determine the number of “pledged delegates” that are expected to vote for their assigned candidate at the Democratic National Convention, held the summer before the general election. For decades, delegate allocations were determined by governors, mayors and other Democratic Party leaders. That changed after a disastrous Democratic convention in 1968, when thousands of anti-Vietnam War protesters clashed with Chicago police.

Long story short: The Democratic Party picked Vice President Hubert Humphrey as the nominee, who hadn’t run in any primaries, but capitalized on President Lyndon Johnson’s abandoned bid and the delegates who were uncommitted after the assassination of Sen. Robert Kennedy. Humphrey was seen as maintaining support for the war; the vice president’s nomination upset many Democratic voters, and he was defeated by Richard Nixon.

In 1972, the party gave regular voters more influence by using primaries and caucuses to distribute pledged delegates. Today, delegates are distributed proportionally among the candidates who meet a 15-percent vote threshold either state-wide or within districts.

The candidate who collects a majority of pledged delegates — at least 1,991 out of 3,979 total — receives the nomination. If the first ballot cast by pledged delegates at the convention does not yield a candidate with the 1,991 majority, then a second vote would take place that includes both pledged delegates and superdelegates.

Superdelegates (about 771 in total) are party leaders who have the freedom to vote for whichever candidate they want, but their influence in the nominating process has changed significantly since the last election.

WATCH: Elizabeth Warren drops out of the 2020 presidential race

What’s changed for superdelegates in 2020?

During the 2016 primary between Sanders and Hillary Clinton, votes by superdelegates factored into a candidate’s total delegate count on the first DNC ballot. Sanders, who is not a registered Democrat and has often critiqued the Democratic establishment (i.e. the kinds of people who become superdelegates), argued that superdelegates backing Clinton gave her an unfair advantage to meet the nomination threshold. Sanders and his supporters pushed the party to adopt new rules restricting superdelegates from voting in the convention’s first ballot.

The party will do that for the first time in 2020, relying on pledged delegate votes from the primaries and caucuses for that first round. If a candidate does not win the 1,991 majority after the first ballot, then superdelegates would vote in the second round.

As an early primary front-runner this year, Sanders argued the candidate with the most pledged delegates entering the DNC should become the nominee even if that person does not ultimately have a majority at the convention. Sanders maintained this position after Biden surpassed his Sanders’ delegate total. In an interview with MSNBC on Wednesday, Sanders said it could be a “disaster” for the Democratic electorate if the nominee were determined by superdelegates on the second vote.

What is a contested convention, and what is the likelihood it could happen this year?

The terms “contested” convention and “brokered” convention are often used interchangeably, but brokered convention is an outdated term used to describe a process in which party leaders would have a series of negotiations to determine a presidential nominee.

A contested convention is the more modern process in which candidates attempted to resolve potential delegate conflicts before casting the first ballot at the party convention. If neither Biden nor Sanders gains a majority of pledged delegates by the time of the final Democratic caucuses in June, then either candidate may try to court delegates to their side.

For example, the delegates accrued by candidates who have left the race, like Sen. Amy Klobuchar and former Mayor Pete Buttigieg, may be “released” and are free to vote for one of the front-runners, said Brooks D. Simpson, a history professor at Arizona State University.

The additions from former candidates could amount to more than 100 delegates who could tip the scales for one candidate, giving them a majority to win the first round of convention voting.

If, after the first ballot, neither candidate wins a majority, delegates and superdelegates would have multiple rounds of voting until a nominee is chosen.

A genuinely contested convention hasn’t happened since 1976, in a match-up between Republican candidates Gov. Ronald Reagan and President Gerald Ford, with Ford prevailing. A convention with multiple rounds of delegate voting hasn’t happened since 1952.

Though it’s possible to get a contested convention in 2020, that outcome was more probable when a larger group of Democratic candidates were winning delegates — possibly hindering anyone’s ability to reach 1,991. At this stage, however, it’s less likely.

“I think there are enough delegates remaining for one of the top two candidates to win,” said Andrew Smith, director of the University of New Hampshire Survey Center.

With early success in Iowa, New Hampshire and Nevada, Sanders emerged as the candidate with the largest delegate count. But Biden’s wins in South Carolina and on Super Tuesday changed the landscape of the race, giving him the lead delegate count.

“The migration of voters to Biden has been astonishing,” Simpson said. “I think Sanders has really got his work cut out for him.”

But the race is still close enough and far from over. The next contests across 13 states and U.S. territories through the end of March will be crucial for both campaigns.

Why are people worried about having a contested convention?

Political parties want to present a united front in supporting their nominee and nominees who come out of heavily divided conventions struggle to win the general election. For Democrats in particular, memories of the tense 1968 convention still haunt the party, said Anthony Corrado, a government professor at Colby College. “The parties want their convention to essentially be a three-day commercial to launch the general election,” Corrado said. “They don’t want to show off a divided party.”

Party discord is largely regarded as one of the factors contributing to Trump’s 2016 election. Driven by fears of repeating the outcome of that campaign cycle, party leaders and Democratic voters have fretted over which candidate is most electable — and yet the dramatic divisions remain. Voters under the age of 45 who are more liberal are more likely to back Sanders and his positions on issues like health care and climate change. Biden’s supporters, who are generally 65 and older, view the former vice president as the better candidate to take on Trump in November.

“It almost doesn’t matter if there’s a contested convention or not. The divisions within the Democratic Party are going to show up anyway,” Smith said. “They are not going to be tapered over, and I don’t think Biden is really in a good position to do that.”

Leading up to the convention and into the fall, the key question will be whether the common goal of defeating Trump will be enough to temporarily heal those wounds and motivate voters across the Democratic spectrum to unite behind the chosen nominee.

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