Donald Trump looked to cement his hold on the Republican presidential nomination with a sweep of Super Tuesday primaries, all but kicking off the formal campaign against President Joe Biden and an attempted shock return to the White House.

Fifteen states and one US territory were holding nomination contests, offering a huge slew of delegates that in normal election years often sees one candidate emerge from a crowded field.

This time, Trump's sole remaining challenger Nikki Haley is already only just hanging on, giving the scandal-plagued ex-president a chance to bury her for good.

"I think that Trump will win overall, nationally," said supporter Richard Peterson, 72, as he voted in Quincy, Massachusetts.

The expected Trump surge comes a day after the Supreme Court denied a bid by a handful of states to keep him off the ballot due to his attack on the 2020 election, when he refused to concede defeat to Biden and sparked a mob assault on the US Capitol.

A Trump-Biden rematch in November now looks all but certain.

Biden, 81, is also on the ballot in Democratic primaries on Tuesday, but is only being challenged by little-known outsiders, making his re-nomination fight a formality.

On Thursday, the Democrat will address the nation in the State of the Union address to Congress, a high-profile opportunity to lay out his campaign platform and attack Trump, 77.

The lineup of states up for grabs Tuesday included the giant battlegrounds of California and Texas, offering 70 percent of the delegates a candidate needs to be named the presumptive nominee.

Trump would not be able mathematically to close out the contest Tuesday but he expects to be anointed by March 19 at the latest, according to his campaign.

  Will moderates back Trump? 

 "We've been sort of in a rocket, we've been launching like a rocket, to the Republican nomination," Trump told supporters at a weekend rally in Richmond, Virginia.

He made clear he is already looking past the primary to the autumn presidential election itself, telling the crowd: "The biggest day in the history of our country is November 5."

Trump is backed by a passionate core of supporters ready to ignore his attack on the 2020 election and four ongoing criminal cases. However, Haley's campaign has raised questions over whether middle-of-the-road Republicans will drift away from Trump in November's election.

In Arlington, Virginia, near Washington, 64-year-old Sarah said she had previously backed Trump but this time was opting for the more moderate Haley.

"I cannot in good faith vote for him again," the retired nutritionist said. Haley "is the right person to unify us."

Haley, 52, lost the early nominating states to Trump by wide margins, but has vowed to remain in the presidential contest at least through Super Tuesday.

  Haley's 'chaos' warning 

 Haley has been making an electability argument -- that the public has rejected Trump's divisive brand in almost every vote since 2016 and would do so again in November.

She also warns of the "chaos" surrounding a candidate who in just the last few months has been labeled an insurrectionist by a federal judge and found liable for sexual assault and business fraud running to hundreds of millions of dollars.

Trump -- who denies all wrongdoing -- also faces the threat of jail time from multiple federal and state felony charges, mostly for allegedly trying to cheat in or steal the 2020 election.

Trump has spent nine days in court this year alone, and complains his prosecutions are keeping him from the campaign trail -- although he has turned court appearances into part of his fundraising campaign.