Former US Ambassador to the United Nations Nikki Haley will suspend her presidential campaign on Wednesday, according to a source familiar with her plans.
This means that former President Donald Trump is guaranteed the nomination by the Republican Party and will again face Democrat Joe Biden in the November elections.
Ms Haley will not express support for Mr Trump in her speech, according to the source who confirmed the news. She will urge him to try to win the support of her supporters, which include a significant portion of moderate Republicans and independent voters, according to the same source.
Ms. Haley's decision to withdraw from the race comes a day after Super Tuesday voting saw Mr. Trump convincingly win 14 of the 15 races for the Republican nomination.
She stayed on the campaign trail longer than any of Mr. Trump's Republican rivals, but at no point did she pose a serious threat to the former president.
Haley, 52, had secured the support of influential donors seeking to block Mr Trump's candidacy, particularly after performing well in several debates in which former President Trump did not participate.
But she failed to win over a sufficient number of conservative voters in the face of Mr. Trump's dominance.
But the fact that support for him was most visible among moderate Republicans and independents could be an indication that Mr. Trump's style of politics could present challenges to him in the Nov. 5 general election.
Mr. Biden, on the other hand, faces his own issues, including widespread concern about his age.