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 flagbearers, 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 -- a US 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 to propel him and his Make America Great Again movement back into the cauldron of a presidential race.

The delegates -- members of party leadership and other loyalists -- 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 -- the Pacific island state where polls were to close hours later on Tuesday -- were offering a combined 161 delegates on the Republican side, and unopposed Trump needed 137 of those to put the race mathematically beyond reach.

Trump's remarkable 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.

Trump, 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.

"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."

- 'Horror show' -

Trump is campaigning on sweeping reform of what he calls Biden's "horror show" immigration policies, despite successfully pressuring Republicans to block the toughest package of border security negotiated in Congress for decades.

The issue has become a flashpoint in Georgia -- which was long reliably Republican but has become more competitive and is now seen as crucial to any candidate's White House ambitions -- due to the recent murder of nursing student Laken Riley, allegedly by an undocumented migrant.

"We're looking at open borders and we're looking at inflation. Those two issues (have) already had people pretty agitated in Georgia," Republican Brad Raffensperger, the state's top elections official, told Fox News.

"But that brutal murder... just really took it to a whole different level. People are furious here in Georgia."

The contests have renewed scrutiny of Trump's effort to overturn the 2020 election in Georgia -- a southern state he lost to Biden by fewer than 12,000 votes -- as he gears up for a third White House run.

The push led to one of the four indictments he faces, setting the stage for a year of unprecedented drama as the 77-year-old tries to juggle multiple court appearances and another election campaign.

The first Republican presidential candidate to lose Georgia in almost three decades, Trump claimed foul play but several recounts and numerous lawsuits failed to turn up any evidence of significant voter fraud anywhere in the country.

On the Democratic side, Biden, like most incumbents, has faced a relaxed primary season -- easily seeing off two candidates who have consistently polled in single digits.

But he faces concerns about his age and issues like inflation, as well as low approval ratings, elements Trump's team has sought to seize on during the campaign.

Biden essentially raised the curtain on his re-election campaign last week during his set-piece State of the Union speech in which he sought to reassure voters about his age and launched a scorching attack on his "dangerous" election rival.

Polls show Americans are largely dreading a rematch between the two men.