Nikki Haley suspended her presidential campaign on Wednesday after being soundly defeated across the country on Super Tuesday.
The move by the former US Ambassador to the United Nations leaves former president Donald Trump as the last remaining major candidate for the 2024 Republican nomination.
Ms Haley announced her decision after suffering a string of significant losses.
"The time has now come to suspend my campaign," Haley said in a speech in Charleston, South Carolina. “I have no regrets."
Haley didn't endorse Mr Trump, who continued his march towards the Republican nomination with wins in 14 of the 15 Super Tuesday Republican primary contests.
Instead, she encouraged him to earn the support of the coalition of moderate Republicans and independent voters who supported her.
"It is now up to Donald Trump to earn the votes of those in our party and beyond it who did not support him. And I hope he does that," she said. "At its best, politics is about bringing people into your cause, not turning them away. And our conservative cause badly needs more people."
Reports began to emerge earlier on Wednesday that Haley was dropping out of the race, after the candidate made no public appearances following her defeats.
Ms Haley, who logged her only victory of the day in Vermont, spent the night huddled with staff watching returns near her South Carolina home.
Trump on Tuesday night declared that the GOP was united behind him, but Haley spokesperson Olivia Perez-Cubas said that the support Ms Haley received in Vermont and elsewhere showed that the GOP is far from unified.
She said: "Unity is not achieved by simply claiming 'we're united.
“Today, in state after state, there remains a large block of Republican primary voters who are expressing deep concerns about Donald Trump.
“That is not the unity our party needs for success."
Ms Haley had already entered Super Tuesday as a huge underdog in the Republican presidential nomination contest.
Mr Trump has tapped into a current of dissatisfaction about the state of the country that he has amplified at every opportunity.
It is one that could tip the scales in the rematch with Mr Biden, who beat him in 2020.
Republican voters who went to the polls on Tuesday exhibited a deep pessimism about the economy that extends beyond Mr Trump’s base of supporters to many moderate and swing voters that could help determine the election, according to exit polls conducted by Edison Research.
Voters also said they are increasingly alarmed by the situation at the US border with Mexico, with many calling it their top voting issue even while living hundreds or even thousands of miles away.
“There is widespread dissatisfaction with Biden and the way things are going in the county today,” said Mark Baldassare, statewide survey director at the nonpartisan Public Policy Institute of California.
“The focus on immigration and economy is what caught my attention the most,” he added.
Mr Biden had a good day too, racking up hundreds of delegates as he cruises toward the Democratic nomination.
