Nikki Haley suspends presidential campaign, leaving Donald Trump ...

Nikki Haley has suspended her US presidential campaign but stopped short of endorsing Donald Trump, arguing he needs to "earn the votes" of Americans who don't support him.

Ms Haley had been the former president's only remaining rival for the Republican nomination but was defeated in all but one of yesterday's Super Tuesday elections.

Speaking in her home state of South Carolina, the former United Nations ambassador said she had "no regrets".

"I am filled with the gratitude for the outpouring of support we've received from all across our great country, but the time has now come to suspend my campaign," she said.

"I said I wanted Americans to have their voices heard. I have done that."

Ms Haley congratulated the former president but did not publicly back him as some other Republican candidates did when they dropped out.

"It is now up to Donald Trump to earn the votes of those in our party and beyond it who did not support him," she said.

"And I hope he does that.

"At its best, politics is about bringing people into your cause, not turning them away ... this is now his time for choosing."

Ms Haley, a former South Carolina governor and UN ambassador, was Mr Trump's first significant rival when she jumped into the race in February 2023.

She spent the final phase of her campaign aggressively warning the GOP against embracing Mr Trump, whom she argued was too consumed by chaos and personal grievance to defeat US President Joe Biden in the general election.

Her departure clears Mr Trump to focus solely on his likely rematch in November with Mr Biden. The former president is on track to reach the necessary 1,215 delegates to clinch the Republican nomination later this month.

Mr Trump posted to his social media platform, Truth Social, just before Ms Haley's announcement, saying she was "TROUNCED last night, in record setting fashion".

A fundraising committee supporting the former president described the contest as a "hard fought race", and argued Republicans now needed to unite ahead of the general election against president Joe Biden.

Ms Haley's defeat marks a painful, if predictable, blow to those voters, donors and Republican Party officials who opposed Mr Trump and his fiery brand of "Make America Great Again" politics.

She was especially popular among moderates and college-educated voters — constituencies that will likely play a pivotal role in the general election.

It's unclear whether Mr Trump, who recently declared that Ms Haley's donors would be permanently banned from his movement, can ultimately unify a deeply divided party.

Mr Biden sought to appeal to Ms Haley's supporters in a statement that commended her for being "willing to speak the truth" about Mr Trump.

"Donald Trump made it clear he doesn't want Nikki Haley's supporters. I want to be clear: There is a place for them in my campaign," he said.

"I know there is a lot we won't agree on.

"But on the fundamental issues of preserving American democracy, on standing up for the rule of law, on treating each other with decency and dignity and respect, on preserving NATO and standing up to America's adversaries, I hope and believe we can find common ground."

Nikki Haley's supporters could play a crucial role in the general election.(ABC News: Brad Ryan)

Ms Haley leaves the 2024 presidential contest having made history as the first woman to win a Republican primary, having beaten Mr Trump in the District of Columbia on Sunday and Vermont on Tuesday.

She had insisted she would stay in the race through Super Tuesday, and crossed the country campaigning in states holding Republican contests.

Ultimately, however, she was unable to knock Mr Trump off his glide path to a third straight nomination.

Ms Haley's allies note that she exceeded most of the political world's expectations by making it as far as she did.

She had initially ruled out running against Mr Trump in 2024. But she changed her mind, and ended up launching her bid three months after he did, citing among other things America's economic troubles and the need for "generational change".

Ms Haley, 52, later called for competency tests for politicians over the age of 75 — a knock on both Mr Trump, who is 77, and Mr Biden, who is 81.

Her candidacy was slow to attract donors and support, but she ultimately outlasted all of her other GOP rivals, including Florida Governor Ron DeSantis, former vice-president Mike Pence and Senator Tim Scott, her fellow South Carolinian whom she appointed to the Senate in 2012.

She gained popularity with many Republican donors, independent voters and the so-called "Never Trump" crowd, even though she criticised the criminal cases against him as politically motivated and pledged that, if president, she would pardon him if he were convicted in federal court.

As the field consolidated, she and Mr DeSantis battled it out through the early-voting states for who would come a distant second to Mr Trump.

The two went after each other in debates, ads and interviews, often more directly than they went after Mr Trump.

Mr DeSantis and Ms Haley would end up debating each other after Mr Trump refused to participate.(Reuters: Mike Segar)

The campaign's focus on foreign policy following Hamas's surprise attack on Israelis in October tilted the campaign into Ms Haley's wheelhouse, giving her an opportunity to showcase her experience from the UN, tying the war to her conservative domestic priorities and arguing that both Israel and the US could be made vulnerable by what she called "distractions".

However, Ms Haley was slow to criticise her former boss directly.

As she campaigned across early states, she often complimented some of Mr Trump's foreign policy achievements, but gradually inserted more critiques into her campaign speeches.

She argued Mr Trump's hyper-focus on trade with China led him to ignore security threats posed by a major US rival, and warned that weak support for Ukraine would "only encourage" China to invade Taiwan — a viewpoint shared by several of her GOP rivals — even as many Republican voters questioned whether the US should send aid to Ukraine.

Ms Haley takes a selfie with supporters in Raleigh, North Carolina on Saturday.(AP Photo: Chuck Burton)

In November, Ms Haley — an accountant who had consistently touted her lean campaign — won the backing of the political arm of the powerful Koch network.

AFP Action blasted early-state voters with mailers and door-knockers, committing its nationwide coalition of activists and virtually unlimited funds to helping her defeat Mr Trump.

With the former president refusing to participate in primary debates, Ms Haley went head-to-head with Mr DeSantis in a single debate, displaying a combative style that seemed to sit poorly with even those committed to support her in the Iowa caucuses. She would finish third.

Ms Haley's name emerged as a possible running mate for Mr Trump, who reportedly asked allies what they thought of adding her to his possible ticket. As she appeared to gain ground, some of Mr Trump's backers worked to tamp down the notion.

While Ms Haley initially notably declined to rule out the possibility, she said while campaigning in New Hampshire in January that serving as "anybody's vice-president" is "off the table".

After Mr DeSantis exited the campaign following Mr Trump's record-setting win in the Iowa caucuses, Ms Haley hoped that New Hampshire voters would feel so strongly about keeping the former president away from the White House that they would turn out to support her in large numbers.

"America does not do coronations," she said in Franklin on the eve of the New Hampshire primary.

"Let's show all of the media class and the political class that we've got a different plan in mind, and let's show the country what we can do."

But she would lose New Hampshire, and then refused to participate in Nevada's caucuses, arguing the state's rules strongly favoured Mr Trump.

She instead ran in the state's primary, which didn't count for any delegates for the nomination.

She still finished a distant second to "none of these candidates", an option Nevada offers to voters dissatisfied with their choices and used by many Mr Trump supporters to oppose her.

She had long vowed to win South Carolina, but backed off that pledge as the primary drew nearer.

She crisscrossed the state that twice elected her governor on a bus tour, holding smaller events than Mr Trump's less frequent rallies and suggesting she was better equipped to beat Mr Biden than him.

She lost South Carolina by 20 points, and Michigan three days later by 40. The Koch brothers' AFP Action announced after her South Carolina loss that it would stop organising for her.

But by staying in the campaign, Ms Haley drew enough support from suburbanites and college-educated voters to highlight Mr Trump's apparent weaknesses with those groups.

Ms Haley has made it clear that in addition to not wanting to serve as Mr Trump's vice-president, she would not run on a third-party ticket arranged by the group No Labels.

She leaves the race with an elevated national profile that could help her in a future presidential run.

In recent days, she also backed off a pledge to endorse the eventual Republican nominee that was required of anyone participating in party debates.

"I think I'll make what decision I want to make," she told NBC's Meet the Press.

AP/ABC