Explainer-How Super Tuesday could be Haley's last chance to stop Trump

By Costas Pitas

(Reuters) - Fifteen states and one U.S. territory hold Republican nominating contests on Super Tuesday, the biggest day of votes when more than a third of delegates will be assigned in the battle to decide if Nikki Haley can stop frontrunner Donald Trump from winning the Republican presidential nomination.

Democrats also head to the polls for primaries and caucuses around the nation on March 5, but President Joe Biden is a shoo-in for the nomination with only one long-shot challenger remaining.

Here are the key details about Super Tuesday:

WHAT IS SUPER TUESDAY?

Super Tuesday describes the day in the U.S. 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).

It is also the final day for Democrats in Iowa to mail in their ballots in that state's nominating contest 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 Election Day and received by March 12.

In addition, some states hold "open primaries" that allow registered voters to choose whether to cast their ballot 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, though.

Polls show Trump to be an overwhelming favorite in California and Texas, as well as in states such 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 easily swept all five Republican nominating contests thus far in Iowa, New Hampshire, Nevada, the U.S. Virgin Islands and South Carolina. He has repeatedly called on Haley to drop out, which would set up a rematch of the 2020 election against Biden that polls show many Americans don't want.

Haley, who served as United Nations ambassador under Trump, has defiantly 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.

(Reporting by Costas Pitas, additional reporting by James Oliphant; Editing by Colleen Jenkins and Alistair Bell)