Florida Governor Ron DeSantis - once tipped as Donald Trump's biggest challenger for the Republican presidential nomination - has suspended his campaign.
It comes after Mr DeSantis only narrowly beat his nearest rival, Nikki Haley, to second place in the Republican Party's Iowa caucuses, far behind Mr Trump, who scored a record-breaking victory.
Mr DeSantis announced his decision in a video posted on the social media platform X, formerly Twitter, in which he endorsed Mr Trump for president.
He said, following the result and "deliberation" with his campaign team, that he could no longer see a "clear path to victory" in the Republican race.
"If there was anything I could do to produce a favourable outcome, more campaign stops, more interviews, I would do it," he said.
"But I can't ask our supporters to volunteer their time and donate their resources. We don't have a clear path to victory.
"Accordingly, I am today suspending my campaign."
Giving his endorsement to Mr Trump, he added: "It's clear to me that a majority of Republican primary voters want to give Donald Trump another chance.
"He has my endorsement because we can't go back to the old Republican guard of yesteryear, a repackaged form of warmed-over corporatism that Nikki Haley represents."
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The move comes two days before the New Hampshire primary - where Ms Haley is tipped to perform well.
Mr DeSantis, who campaigned heavily in Iowa, had reportedly shifted his campaign team's focus to the next primary, in Ms Haley's home state of South Carolina, prior to Sunday's announcement.
Ms Haley declared "may the best woman win" in response to his exit from the race, promising to "fight all the way until the last second".
Mr Trump did not mention Mr DeSantis's withdrawal during a short speech to Republican voters in Rochester, New Hampshire, on Sunday.
However, his campaign team said in a statement they were "honoured" by Mr DeSantis's endorsement, before calling on Republicans to "rally behind" Mr Trump.
A proponent of hard-right and "anti-woke" policies, Mr DeSantis had sought to position himself as an alternative to Mr Trump.
He had attempted to cast himself as a politically successful heir to the Make America Great Again (MAGA) movement without Mr Trump's baggage - a move which saw him labelled "Trump-lite" by his opponents.
Mr DeSantis also carried into the race a strong election record, having achieved a blowout 2022 re-election win in Florida, which had for decades been one of the most tightly divided states in the nation.
But he struggled to peel away enough of Mr Trump's support, while Ms Haley, a former UN ambassador, mopped up moderate Republican voters.
His campaign team reportedly spent tens of millions of dollars going door-to-door to promote Mr DeSantis in Iowa.
However, while he came in second, he was still 30 percentage points behind Mr Trump, who captured more than 50% of the roughly 110,000 votes.