Ohio Governor John Kasich has dropped out of the presidential race after struggling to gain traction against Republican front-runner Donald Trump.
Mr Kasich did not have a path to secure the nomination outright, but pledged to lobby for his candidacy during the Republican convention in July.
Mr Trump holds a commanding lead and is closing in on the nomination.
His likely opponent will be Democratic front-runner Hillary Clinton, who lost the Indiana primary to Bernie Sanders.
It was a surprise win for the Vermont senator who continues to attract huge crowds to his rallies, but his opponent has an almost insurmountable lead.
Speaking to CNN about taking on Mr Trump, Mrs Clinton said he was a “loose cannon” who had ran a “negative, bullying” campaign.
The New York businessman has made a series of controversial remarks ever since he launched his White House bid by labelling Mexicans as rapists and criminals.
Several senior Republicans said on Wednesday they will not back him, with some saying they would prefer to vote for Mrs Clinton.
Trump nomination divides Republicans
Mr Kasich’s announcement, which will be made later but has been leaked to the US media, clears Mr Trump’s path although he was never a significant threat and only won his home state.
Earlier on Wednesday, he had released a Star-Wars themed advert describing himself as the “only hope” against Donald Trump.
Analysis – Anthony Zurcher, BBC North America reporter
The race for the Republican presidential nomination has taken more than a year to unfold, but in a flash it is over.
Ted Cruz’s withdrawal from the race Tuesday night meant John Kasich’s long-shot path to the nomination – deadlocked delegates in a contested convention turning to him as a compromise candidate – was definitively closed.
The Ohio governor, once thought to be the saviour of the moderate, establishment wing of the Republican Party, could have soldiered on, but with little money and no hope of winning, such a course bordered on the absurd.
Although Mr Trump had effectively sewn up the nomination regardless of what Mr Kasich decided to do, his withdrawal does have one benefit. Now the New York businessman will not have to make even pro forma campaign stops in California, which holds its primary on 6 June.
Just last week the front-runner faced massive protests while attending the state’s Republican convention. California looked to be a powder keg for Mr Trump in the coming weeks. Thanks to Mr Kasich, it has been defused.
Texas Senator Ted Cruz dropped out of the race on Tuesday after losing to Mr Trump in the Indiana primary.
It is now certain Mr Trump will have the 1,237 delegates needed to become the nominee before the July convention in Cleveland, Ohio.
Mr Kasich had been widely seen as the most moderate and electable Republican candidate but this did not garner him enough support among Republican primary voters.
Republicans are now divided over whether to support Mr Trump as the Republican nominee.
“If we nominate Trump, we will get destroyed… and we will deserve it,” South Carolina Senator Lindsay Graham said on Tuesday.
Mr Kasich’s named has been floated as a possible vice presidential pick but he has denied that he would accept it..
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