Atlanta, Georgia - President Joe Biden and rival Donald Trump each won enough delegates Tuesday to clinch their party nominations in the 2024 presidential race, networks projected, all but assuring a rematch and setting up one of the longest election campaigns in US history.

The results in four statewide elections Tuesday, the latest in the months-long march to determine the Democratic and Republican party nominees, were essentially a foregone conclusion as incumbent Biden and former president Trump had already seen off all primary challengers.

Biden crossed the threshold of 1,968 delegates needed when he won Georgia, the swing state where Trump faces trial over an alleged conspiracy to steal the last election.

Trump's victory in Washington helped him secure the 1,215 delegates needed to earn the Republican nomination and propel him back into the cauldron of a presidential race.

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The delegates will attend the national conventions, where they formally select their party's presidential nominee.

As the pair now head for a rematch of their 2020 showdown, Biden laid into his challenger in a statement.

"I am honored that the broad coalition of voters representing the rich diversity of the Democratic Party across the country have put their faith in me once again to lead our party – and our country – in a moment when the threat Trump poses is greater than ever," Biden said, assailing his rival's "campaign of resentment, revenge, and retribution."

Georgia, Mississippi, Washington, and Hawaii offered a combined 161 delegates on the Republican side, and unopposed Trump needed 137 of those to put the race mathematically beyond reach.

Trump's sweep of nearly all GOP state primaries to date led him to essentially secure the nomination far earlier than most candidates in previous campaigns, and it assures an extremely lengthy, nearly eight-month slog for the White House being contested by the two oldest men ever to begin their presidencies.

The 77-year-old, who faces multiple criminal indictments in cases that to date have failed to derail his 2024 campaign, insisted in a victory statement that the Republican Party is strong and united behind him.

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"We are now, under Crooked Joe Biden, a Third World Nation, which uses the Injustice System to go after his political opponent, ME!" he wrote on his Truth Social media platform.

"But fear not, we will not fail, we will take back our once great Country."

Biden, meanwhile, had a fairly straightforward primary season complicated by his unconditional support for Israel's brutal assault on Gaza, which has alienated large swathes of his base and led to a campaign to register dissatisfaction by selecting the uncommitted option on ballots.

Polls show Americans are largely dreading the Trump-Biden rematch.