Donald Trump will march to the threshold of a third Republican presidential nomination this week as millions of Americans vote in "Super Tuesday" -- a critical milestone in his party's lopsided 2024 race for the White House.

Normally the most consequential part of the campaign before the election itself, Super Tuesday decides Republican and Democratic primaries in the giant states of California and Texas, as well as more than a dozen other battlegrounds.

But the usual drama will largely be absent this year, with the door expected to swing firmly shut on Trump's only remaining challenger, Nikki Haley, and clear the way for a repeat of his 2020 match-up with Joe Biden.

The president is virtually unopposed in the Democratic contest and will look to keep an insurmountable lead against two single-digit challengers.

Haley lost the early nominating states by wide margins, and pundits give her little chance of catching up to Trump, who claims his opponent has "no traction" and will lose every Super Tuesday state.

"If every single conservative, Republican and Trump supporter in these states shows up on Super Tuesday, we will be very close to finished with this primary contest," Trump said in a campaign message ahead of the big vote.

"Republicans will then be able to focus all of our energy, time and resources on defeating crooked Joe Biden, the worst president in the history of our country."

Fifteen states and one territory -- American Samoa -- make up Tuesday's map, and hopefuls can bag 70 percent of the delegates they need to be named the presumptive nominee.

Trump cannot close out the contest on that night but expects to be anointed by March 19 at the latest.

- Post-Trump Republicans -

Haley, who warns voters that nominating Trump spells general election defeat for Republicans, has plenty of cash on hand but says she has not decided if she will continue beyond Tuesday.

Her goal is to make America a better country, she says, although many analysts suspect she is building for a more realistic run in the post-Trump party of 2028.

Others have speculated that she is hanging on until the Republican National Convention in July in case delegates have second thoughts about formally nominating Trump as his criminal cases close in.

The tycoon has spent nine days in court this year, complaining that his prosecutions -- mostly involving allegations of cheating in elections -- are keeping him from the campaign trail.

As he makes his case for reelection in a televised address Tuesday at his south Florida beach club, his lawyers will be preparing their own arguments for the start of his New York trial for 2016 campaign finance violations.

The whiff of scandal that follows Trump is seen as a turn-off for middle-of-the-road Americans, and a chunk of moderate Republicans have repudiated him in each primary contest.

Analysts have questioned whether he has the ability or will to broaden his appeal beyond his loyal base and parlay his popularity in the primaries into a second term in office.

"Crucial battleground states either have a fairly equal proportion of voters registered as Democrats and Republicans, or lean Democratic," said political analyst Gerard Filitti, senior counsel at Jewish-focused legal think tank The Lawfare Project.

"If only 60 percent of Republicans vote for Trump and he fails to attract the majority of independents -- as is suggested by his polling to date -- barring an unusually low turnout for Biden, the math is not there for him to win."

But Biden also faces divisions among Democratic supporters, and a New York Times survey published on Saturday flagged waning support among normally reliable constituencies like blue-collar workers and non-white voters.

The nationwide poll of registered voters found that if the election were held today, 48 percent would choose Trump with 43 percent for Biden.