Washington DC - 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 an exceptionally high-risk move.

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

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.

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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.

The Democratic Party has announced that they will hold a "transparent and orderly" process to pick a new nominee, the party chair said Sunday after President Joe Biden announced he was dropping out of the White House race.

"In the coming days, the party will undertake a transparent and orderly process to move forward as a united Democratic Party with a candidate who can defeat Donald Trump in November," chair Jaime Harrison said in a statement praising Biden.

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 was pulling out.

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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.