Americans in 15 states head to the polls on Super Tuesday – the biggest day so far of the 2024 US presidential election cycle – but the outcome is not expected to be much of a surprise.

Donald Trump and Joe Biden are all but certain to win the nominations for their respective parties, setting up a potential presidential rematch.

Trump has dominated the Republican campaign and swept all but one of the contests so far against rival Nikki Haley, although she’s refusing to back down.

Meanwhile, President Biden is expected to be a shoo-in against Democrat opponents Dean Phillips and Marianne Williamson.

Super Tuesday is the biggest voting day of the US primary season, with 15 states and one territory (American Samoa) holding contests for Republican and Democrat candidates.

Swift implores fans

As voters headed to the polls, American superstar Taylor Swift took time from her hectic concert schedule to urge her millions of fans to have their say.

“Today, March 5, is the presidential primary in Tennessee and 16 other states and territories. I wanted to remind you guys to vote the people who most represent YOU into power,” she said in an Instagram story.

“If you haven’t already, make a plan to vote today.

“Whether you’re in Tennessee or somewhere else in the US, check your polling place and times at Vote.org.”

Enrolment numbers historically spike after a Swift callout. Her first foray into American politics was in in 2018, when she urged fans in her home state of Tennessee to vote Democrat.

In 2020, Swift endorsed Biden, but it’s yet to be seen if she will wade any deeper into the 2024 contest.

She has also denounced former president Trump, a Republican.

The 14-time Grammy award winner, whose ongoing Eras tour is the world’s highest-grossing concert tour with more than $US1 billion ($1.5 billion) in ticket sales, has 282 million followers on Instagram.

During an appearance on NBC’s Late Night with Seth Meyers last week, Biden joked about conspiracy theories that he and Swift were in “cahoots”.

Super Tuesday contest

While Trump cannot win enough delegates on Tuesday to formally clinch the Republican nomination, another dominant performance would essentially end any remaining sliver of suspense.

Tuesday’s contests will award more than a third of Republican delegates – and more than 70 per cent of the number needed to secure the nomination.

A third consecutive nomination for Trump would set up a rematch with Biden in November’s election.

Biden is expected to win Tuesday’s Democratic contests easily, though activists opposed to his Israel policy are calling on Muslim Americans and progressives to vote “uncommitted” in Minnesota in protest.

Haley, a former UN ambassador under Trump, has faced mounting questions about how long she will continue her long-shot campaign, particularly after losing her home state of South Carolina 10 days ago.

She has made no promises beyond Super Tuesday, and her campaign has not scheduled any public events on Tuesday or beyond.

“As much as everybody wants to go and push me out, I’m not ready to get out yet,” she told Fox News in an interview on Tuesday morning.

Trump was leading Haley in every Super Tuesday state where public polling data was available, according to poll tracking website 538. In California and Texas, which together account for more than 300 delegates, Trump was ahead by an average of more than 50 per cent.

But Haley allies see a narrow window of opportunity to eke out a win in states including Virginia, Massachusetts and Vermont, which have more of the wealthy, college-educated voters who tend to support her candidacy.

Those three states are also among several on Super Tuesday that do not require primary voters be registered Republicans. Independent and moderate voters have favoured Haley over Trump in early voting states, according to exit polls from Edison Research.

Americans in 15 states vote on Super Tuesday. Photo: Getty

Asked if he would reach out to Haley after Tuesday’s results, Trump said his focus was on Biden.

“I think we’re going to win every state tonight,” he told Fox News in a separate interview.

Trump’s advisers have said they expect him to eliminate Haley mathematically no later than March 19; at that point, two-thirds of the states will have voted.

Trump is scheduled to begin his first criminal trial six days later in New York, where he is charged with falsifying business records to conceal hush money payments to a porn star during his 2016 presidential campaign.

Haley scored her first victory on Sunday in Washington, DC, becoming the first woman ever to win a Republican presidential primary.

While Haley has failed to slow Trump’s momentum, her challenge has highlighted some of his potential vulnerabilities in a general election. She has repeatedly noted that she reached 40 per cent in some state contests, arguing that her performance shows independents and moderate Republicans harbour unease about a second Trump term.

The US Supreme Court on Monday ruled that states cannot remove Trump from their ballots based on a US constitutional amendment barring insurrectionists from holding office.

The decision, while a victory for Trump, was also a reminder of his efforts to overturn the 2020 election, which culminated in an attack on the US Capitol on January 6, 2021, by a mob of his supporters.

Trump faces federal and state charges for election interference, though it is not clear whether either case will reach trial before November’s election. In addition to those two cases and this month’s upcoming hush-money trial, he has been indicted on federal charges for retaining classified documents after leaving office.

Trump has pleaded not guilty in every case and claimed without evidence that they are part of a Democratic conspiracy to prevent him from returning to power.

-with AAP