Washington DC - President Joe Biden's woeful performance in the presidential debate against challenger Donald Trump has re-ignited questions about replacing the veteran Democrat at the last minute as the party's flagbearer.

Such a high-risk political U-turn would be unprecedented in modern American election history. Here's a look into how replacing the 81-year-old could be possible.

To designate a party's 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 in August are beholden to him.

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If Biden exits, the delegates would have to find a replacement. That would mean bringing US politics back to the old days, when party bosses jostled to pick a nominee through rounds of deal-making in smoke-filled back rooms and voting in convention halls.

This has not happened since 1960, when John F. Kennedy and Lyndon Johnson fought for the nomination at the Democratic National Convention.

Eight years later, the nomination process swerved into chaos when Johnson, then president, made the shock announcement on March 31, 1968 – 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 protestors and left-leaning delegates angry at the pro-war stance of party-picked candidate Hubert Humphrey.

Since then, conventions have been well-oiled affairs, whose outcomes have been known in advance since they are determined by the primaries.

The closest thing to a potential switch from Biden dates back to 1972, when Senator Thomas Eagleton was forced to step aside as Democratic vice presidential nominee after the convention when it was revealed he was treated for mental illness.

When a candidate suffers such a fate after being officially nominated at the convention, a party's formal governing body, either the Democratic National Committee or Republican National Committee, would nominate a new candidate in an extraordinary session.

A natural – but not automatic – pick to take Biden's place would be his running mate on the 2020 ticket, Vice President Kamala Harris.

Sent in Thursday night to put out the fire after the Democratic president's lackluster performance, the 59-year-old conceded Biden had been "slow to start" the debate 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 are mentioned – might be called on.

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Meanwhile, third-party and Independent candidates have been making the case why it is time to move beyond the two-party system.

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

But in the end, because of the vagaries of the American electoral system, 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.