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By Sam Cabral
BBC News, Washington
A powerhouse donor base that has funded conservative causes for more than two decades says it is swinging behind Nikki Haley's 2024 presidential bid.
Billionaire Charles Koch's network has raised millions in an effort to help Republicans move past Donald Trump.
But as the party's voters prepare to begin picking their preferred candidate in January, the ex-president dominates the rest of the field.
The Koch endorsement of Mrs Haley has the potential to reshape the race.
The former South Carolina governor is still polling roughly 30 points behind the man she once served as United Nations ambassador.
But her headline-grabbing TV debate performances and a recent opinion polling bump, as a once-crowded Republican field begins to thin out, have given hope to those who want an alternative to Mr Trump.
In recent weeks, several high-profile donors have indicated they will back her over Florida Governor Ron DeSantis, who is polling in second place but whose campaign appears to be faltering.
In a memo on Tuesday, Americans for Prosperity Action - the Koch network's political advocacy arm - celebrated Mrs Haley as "a candidate capable of turning the page on Washington's toxic culture - and a candidate who can win".
It added that internal polling and voter outreach shows Mrs Haley performing best in a hypothetical general election match-up with Joe Biden, the Democrats' current likely 2024 nominee.
In a statement from her campaign, Mrs Haley said she was "honoured" by the support of an organisation that boasts "millions of grassroots members".
"AFP Action's members know that there is too much at stake in this election to sit on the sidelines," she wrote. "We have a country to save, and I'm grateful to have AFP Action by our side."
But the Trump campaign dismissed AFP as "the political arm of the China First, America Last movement".
Spokesman Steven Cheung said in a statement on Tuesday that no amount of "shady money" would help Mrs Haley defeat Mr Trump.
Critics have questioned whether the lone woman in the race has the campaign infrastructure to maintain her momentum as election season heats up.
But it is a problem that Tuesday's endorsement - with the advertising, advocacy and voter mobilisation efforts that accompany it - could go a long way in helping to solve.
It is also a big blow to Mr DeSantis, who was most recently endorsed by AFP ahead of his 2022 re-election as governor.
His campaign slammed the decision as evidence of the political establishment "lining up behind a moderate who has no mathematical pathway of defeating the former president".
The Koch network, which has raised more than $70m (£55m) in the 2024 election cycle, is already spending $4m on ads criticising the ex-president, according to AdImpact.
In its Tuesday memo, AFP apologised to DeSantis supporters but warned that a divided primary field would only benefit Mr Trump and "we are entering a time period that demands choices".
It marks the first time the influential organisation has put its thumb on the scale ahead of a Republican presidential primary.
"As many of you know, AFP Action has never engaged in a presidential election before, but... to write a new chapter for our country, we need to turn the page on the past," AFP's memo reads.
"Donald Trump and Joe Biden will only further perpetuate the country's downward spiral in politics."
The Koch network did not back Mr Trump's successful bid for the White House in 2016.
During the Trump years, it scored several major policy victories, including on tax reform, but opposed his stance on government spending and trade issues.
The ex-president and his potent brand of populism have, in turn, helped to diminish the influence people he refers to as "globalist elites" have over the Republican party.
Republicans will begin voting in the 2024 primary in January. Their eventual nominee currently looks likely to face Mr Biden next November.