After absorbing bruising losses in the Super Tuesday primaries, Nikki Haley, the former governor of South Carolina, announced on Wednesday that she was ending her presidential campaign.

In handing the party’s nomination to former President Donald J. Trump, a rival with whom she clashed bitterly by the end, she declined to endorse him. Instead, she challenged him to win over her supporters: a cohort of more moderate Republicans who could not prevent his nomination, but could potentially hurt his prospects in the fall against President Biden.

“In all likelihood Donald Trump will be the Republican nominee when our party convention meets in July. I congratulate him and wish him well,” she said, adding that it was now “his time for choosing.”

Her decision brings to a close the latest struggle over the soul and direction of the Republican Party, a lopsided fight that has stretched back to Mr. Trump’s first presidential run in 2016. She had come to represent a small but not insignificant number of Republicans who saw her as their last, best chance to turn the page on the former president’s divisive and no-holds-barred politics.

For months and in increasingly urgent language, Ms. Haley, who was Mr. Trump’s first ambassador to the United Nations, had tried to paint her former boss as an aging, mentally unsound agent of chaos, unrepentant in his disparagement of veterans and service members and unwilling to remain faithful to the Constitution.

But she made it clear she was no member of the resistance. Nor was she able to loosen Mr. Trump’s grip sufficiently on a vast majority of the party.