Super Tuesday could be former UN Ambassador Nikki Haley's last chance to stop former US President Donald Trump's drive to clinch the 2024 Republican presidential nomination.

Fifteen states and one US territory hold the party's nominating contests on Mar 5, the biggest day of primaries when more than a third of delegates will be assigned to July's Republican National Convention in Milwaukee.

President Joe Biden is a shoo-in for the Democratic nomination when party loyalists vote for delegates to August's Democratic National Convention in Chicago with only two long-shot challengers remaining.

Here are the key details about Super Tuesday:

WHAT IS SUPER TUESDAY?

Super Tuesday describes the day in the US presidential primary cycle when the most states vote.

In the Republican contest, 874 of 2,429 delegates will be up for grabs, including from the two most populous states, California and Texas. At least 1,215 delegates are needed to win the nomination at the Republican National Convention in July.

Contest-by-contest, the Republican delegate counts for the Super Tuesday votes are: Alabama (50), Alaska (29), American Samoa (9) Arkansas (40), California (169), Colorado (37), Maine (20), Massachusetts (40), Minnesota (39), North Carolina (74), Oklahoma (43), Tennessee (58), Texas (161), Utah (40), Vermont (17) and Virginia (48).

About a third of Democratic delegates will also be decided on Mar 5, with nominating contests held in 14 of those 15 states, plus American Samoa. In Alaska, Democrats vote on Apr 6.

Mar 5 is also the final day for Democrats in Iowa to mail in their ballots in that state's caucuses and when results will be announced.

WHEN IS SUPER TUESDAY AND WHAT CAN WE EXPECT?

Super Tuesday is on March 5 this year. With so many states and a territory voting across different time zones, it could take a while for the full results to be clear.

In California, vote-by-mail ballots are valid as long as they are postmarked on or prior to primary election day and received by Mar 12.

In addition, some states hold "open primaries" that allow registered voters to choose whether to cast their ballots in the Democratic or Republican primary, adding a possible layer of unpredictability.

WHY IS IT IMPORTANT?

Haley has no clear path to beating Trump. This could be her last chance to at least slow the former president's path to the nomination.

Opinion polls show Trump to be an overwhelming favourite in California and Texas, as well as in states such as Alabama, Maine and Minnesota. His campaign projects that he will win at least 773 delegates on Super Tuesday and clinch the nomination a week or two later.

Trump has easily swept all six Republican nominating contests in Iowa, New Hampshire, Nevada, the US Virgin Islands, South Carolina and Michigan. He has repeatedly urged Haley to drop out to set up a rematch of the 2020 election against Biden that polls show many Americans don't want.

Haley, a former South Carolina governor who served as UN ambassador under Trump, has vowed to stay in the race. She will crisscross the country with an aggressive travel schedule leading up to Super Tuesday, and her campaign has rolled out a leadership team in Georgia, where voters go to the polls on March 12, a week after Super Tuesday.

Voters, she said after her defeat in South Carolina on Feb 24, "have the right to a real choice, not a Soviet-style election with only one candidate.

"I have a duty to give them that choice," she added.

The Super Tuesday results in North Carolina will be closely watched for signs of each candidate's strength in one of the potential battleground states that could decide the November general election. Trump won the state in 2020 by just over a single percentage point.

North Carolina will award 74 delegates on Super Tuesday. The state allows voters who are unaffiliated with a party to participate in any primary they choose, which could boost Haley's performance given her relative strength with independent voters compared to Trump.