(Reuters) - Former U.S. Ambassador to the United Nations Nikki Haley will suspend her presidential campaign on Wednesday, according to a source familiar with her plans, ensuring that Donald Trump will win the Republican nomination and once again face Democratic President Joe Biden in November’s election.

Haley will give a speech at 10 a.m. local time to address her future in the race, the source said, but she will not be giving an endorsement.

Haley lasted longer than any other Republican challenger to Trump but never posed a serious threat to the former president, whose iron grip on the party's base remains firm despite his multiple criminal indictments.

The rematch between Trump, 77, and Biden, 81 - the first repeat U.S. presidential contest since 1956 - is one that few Americans want. Opinion polls show both Biden and Trump have low approval ratings among voters.

The election promises to be deeply divisive in a country already riven by political polarization. Biden has cast Trump as an existential danger to democratic principles, while Trump has sought to re-litigate his false claims that he won in 2020.

Haley, 52, had drawn support from deep-pocketed donors intent on stopping Trump from winning a third consecutive Republican presidential nomination, particularly after she notched a series of strong performance at debates that Trump opted to skip.

She ultimately failed to pry loose enough conservative voters in the face of Trump's dominance.

But her stronger showing among moderate Republicans and independents - she won unaffiliated voters by a wide margin in New Hampshire and notched almost 40% of the vote in South Carolina - highlighted how Trump's scorched-earth style of politics could make him vulnerable in the Nov. 5 election.

On March 3, she won the Washington, D.C., Republican primary with 62.9% of the vote, versus 33.2% for Trump.

"It’s not surprising that Republicans closest to Washington dysfunction are rejecting Donald Trump and all his chaos," Haley campaign spokesperson Olivia Perez-Cubas said in a statement.

Biden has his own baggage, including widespread concern about his age. Three-quarters of respondents in a February Reuters/Ipsos poll said he was too old to work in government, after already serving as the oldest U.S. president in history.

About half of respondents said the same about Trump.