Ex-White House hopeful Nikki Haley pledges her vote to Donald Trump

Washington: Former presidential hopeful Nikki Haley said Wednesday she will vote for Donald Trump in the US election, but said her one-time bitter rival would have to work to win over her moderate support base as he faces President Joe Biden in November.

The former South Carolina governor, 52, abandoned her White House ambitions in March after suffering several heavy defeats in primary contests - and had not previously indicated whether she would support the man who referred to her repeatedly as "birdbrain."

Despite having made withering criticism of the scandal-plagued ex-president, Haley said at an event for the Washington-based Hudson Institute, a conservative think tank, that she was urging Trump to make nice with her support base.

"Trump would be smart to reach out to the millions of people who voted for me and continue to support me and not assume that they're just going to be with him. And I genuinely hope he does that," she said.

More than two months after ending her White House campaign, Haley has continued to scoop a significant chunk of votes in presidential primary contests - underlining a persistent refusal among a sizable bloc of Republicans to get behind Trump.

"I put my priorities on a president who's going to have the backs of our allies and hold our enemies to account, who would secure the border - no more excuses - a president who would support capitalism and freedom, a president who understands we need less debt, not more debt," she said.

"Trump hasn't been perfect on these policies. I've made that clear, many, many times. But Biden has been a catastrophe. So I will be voting for Trump."

'Zombie campaign'

Trump and Haley traded at times vicious barbs during the state-by-state nominating contests, when the former UN ambassador warned that nominating her one-time boss would spell general election defeat for Republicans.

She said a majority of voters were opposed to a rematch between "chaos" candidate Trump and Biden, two men aged 77 and 81 respectively who she said were past their prime.

"He's getting meaner and more offensive by the day," Haley said ahead of a defeat in her home state of South Carolina in February.

"He's completely distracted, and everything is about him. He's so obsessed with his demons in the past that he can't focus on the future Americans deserve," she told supporters.

Haley criticised Trump's comments attacking NATO nations and referred repeatedly to the many court appearances he has been making as he faces multiple criminal indictments and lawsuits.

"It's not normal to spend $50 million dollars in campaign contributions on personal court cases," Haley said. "It's not normal to threaten people who back your opponent, and it's not normal to call on Russia to invade NATO countries."

Trump returned the compliment by categorically ruling her out of contention to be his running mate.

Trump has failed to make inroads with Haley's moderate backers, and will see her public show of support as a huge boon ahead of the Republican nominating convention in July.

The tenacity of the Haley vote long after she quit the race for the Republican presidential nomination prompted US media to refer to her ongoing presence on the primary stage as a "zombie campaign."

The primaries laid bare Trump's key shortcoming - his lack of appeal among the moderates, independents and voters with college degrees he will need to prevail against Biden.

Meanwhile the Biden campaign had been reaching across the aisle, hoping to attract Haley voters.

It released a TV spot in April targeting suburban battlegrounds with the message: "If you voted for Nikki Haley, Donald Trump doesn't want your vote."