What happens after Biden exit?

US President Joe Biden announced on Sunday that he was withdrawing from the presidential race against challenger Donald Trump, following a dismal debate performance and flagging polling numbers.

The stepping down of the Democratic party's flag bearer at the last minute is unprecedented in modern US electoral history, and a high-risk move.

Here's a look at how replacing the 81-year-old could work.

Chaotic convention?

To designate a formal nominee, delegates from all 50 states attend their party's summer nominating convention to officially anoint a candidate based on primary voting.

Biden overwhelmingly won the primary votes, and the party's roughly 3,900 delegates heading to the convention in Chicago this August are beholden to him.

With Biden exiting, the delegates have to find a replacement. That brings US politics back to the old days, when party bosses jostled to pick a nominee through deal-making in smoke-filled back rooms and endless rounds of voting.

On March 31, 1968, then-president Lyndon Johnson made the shock announcement in the middle of the Vietnam War that he would not seek reelection.

The move turned that year's convention, also in Chicago, into a political crisis with protesters in the streets and left-leaning delegates angry at the pro-war stance of party-picked candidate Hubert Humphrey.

Following that debacle, states more widely embraced the primary process and conventions have become well-oiled affairs whose outcomes are known in advance since they are determined by the primaries.

Who might fill in?

Immediately after the debate, Democrats had circled their wagons around Biden, at least when speaking on the record.

All of that dissolved as time went on, with increasingly senior party leaders publicly questioning the viability of the incumbent's candidacy.

A natural -- but not automatic -- pick to take Biden's place would be his running mate on the 2020 ticket, Vice President Kamala Harris. Biden endorsed her Sunday as he announced he is pulling out.

Sent in to put out the fire after the Democratic president's lackluster performance at the debate, the 59-year-old conceded Biden had been "slow to start" against Trump but had "finished strong."

Otherwise, any of a number of strong Democratic politicians -- Governors Gavin Newsom of California, Gretchen Whitmer of Michigan and Josh Shapiro of Pennsylvania have been mentioned -- might be called on.

Third-party chances?

With Biden dropping out, could a strong third-party hopeful emerge? So far, no independent candidate is posing any danger to the United States's dominant two-party system.

In 1992, Texas billionaire Ross Perot, running as an independent, managed to win nearly 19 percent of the popular vote.

But in the end, because of the way the country's electoral system works, he did not receive a single one of the votes that matter most: those of the 538 members of the Electoral College that ultimately decide the winner.