Joe Biden and Donald Trump eyed unassailable leads in their separate presidential nomination races in primary voting on Tuesday that included Georgia -- a US swing state where Trump faces trial over an alleged conspiracy to steal the last election.

As the pair head for a rematch of their 2020 showdown, Biden, who as the incumbent president has not faced any serious primary challenge, clinched enough delegates to become the presumptive Democratic nominee early Tuesday evening.

"The threat Trump poses is greater than ever," the president said of his Republican rival, who saw off his remaining competition in last week's "Super Tuesday" voting.

Georgia -- along with contests in Hawaii, Washington and Mississippi -- were offering a combined 161 delegates on the Republican side, and the unopposed Trump needs 137 of those to put the race mathematically beyond reach.

Georgia was long reliably Republican but has become more competitive and is now seen as crucial to any candidate's White House ambitions.

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 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 state he lost to Biden by fewer than 12,000 votes -- as he eyes a third run for the White House.

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.

Repeated meddling

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.

He nevertheless meddled repeatedly in Georgia politics, pushing for Raffensperger in a now-infamous taped phone call to "find" enough votes to overturn Biden's victory.

The former president -- who denies all wrongdoing -- is being prosecuted under Georgia's Racketeer Influenced and Corrupt Organizations (RICO) statute, which is usually used to nail down mob figures.

Turnout Tuesday looked light across the state, according to the Atlanta Journal-Constitution, which reported that there were no lines at polling stations.

"If you don't vote today, you don't get to complain about what's on the ballot for November. And there's a lot to complain about," Savannah voter Lynn Weddle told the daily newspaper.

Georgians had cast 284,000 Republican ballots and 155,000 Democratic ballots by the end of early voting on Friday, the paper said. The total early vote in 2020 was 1.2 million.

On the Democratic side, Biden picked up the US-held Northern Mariana Islands Tuesday and was on the primary ballots in three mainland states -- Georgia, Mississippi and Washington -- as well as in the "Democrats Abroad" grouping.

As with most incumbents, he has faced a relaxed primary season -- easily seeing off two candidates who have consistently polled in single digits -- and can also clinch the nomination Tuesday if he wins around 40 percent of the available delegates