Billionaire Charles Koch’s political network is backing the Republican presidential bid of Nikki Haley, the latest sign some big donors are moving her way in a long-shot bid to block former President Donald Trump from winning the GOP nomination.

Americans for Prosperity Action, a conservative group led and partly financed by Koch, said Tuesday that it would support the former South Carolina governor and United Nations ambassador. The organization’s support is a boost to Haley as the campaign enters the final seven weeks before the first nominating contest in Iowa.

“She has what it takes to lead a policy agenda to take on our nation’s biggest challenges and help ensure our country’s best days are ahead,” AFP said. “With the grassroots and data capability we bring to bear in this race, no other organization is better equipped to help her do it.”

As the Republican presidential primary has begun to narrow and Haley has risen in polls, there have been questions about whether she has the campaign infrastructure to take advantage of her momentum. AFP, with a grassroots network of millions of volunteers nationwide and deep pockets, could significantly help her in that regard.

The Koch network didn’t support Trump in his first White House bid in 2016 and the two sides have clashed over his opposition to long-held conservative beliefs on government spending and free trade, as well as over the network’s more welcoming stance on immigration.

Trump’s campaign responded with a statement critical of both AFP and Haley that included the former president’s preferred nickname for someone who once served in his administration. “Americans for Prosperity—the political arm of the China First, America Last movement—has chosen to endorse a pro-China, open borders, and globalist candidate in Nikki ‘Birdbrain’ Haley,” Trump’s spokesman Steven Cheung said.

In recent weeks, Haley has increasingly won the backing of high-profile donors on Wall Street and elsewhere.

Billionaire Ken Griffin, founder of investing giant Citadel and one of the nation’s biggest Republican donors, has said he is thinking about supporting her. His involvement could be significant because he has a history of sending seven- or eight-figure checks to candidates and groups he backs.

Former Trump adviser Gary Cohn and UBS banker Mike Santini co-hosted a recent Haley fundraiser at Cohn’s Upper East Side apartment in Manhattan. Haley also has another fundraising swing through New York planned for early next week.

Ken Langone, the billionaire Home Depot founder, is also contemplating support for Haley and is expected to meet with her next week. “The only person I see who can give Trump a run for his money is Nikki Haley,” he told CNBC on Monday.

The AFP endorsement marks another blow to Florida Gov. Ron DeSantis, who has lost to Haley his grip on claims of being the top alternative to Trump. DeSantis has struggled throughout his campaign and some major donors have backed away from supporting him.

The DeSantis campaign said AFP was helping Trump. “Every dollar spent on Nikki Haley’s candidacy should be reported as an in-kind to the Trump campaign,” spokesman Andrew Romeo said.

Trump maintains a wide lead in polls, including in the early voting states of Iowa, New Hampshire and South Carolina, and has demonstrated fundraising strength through small dollar donations. His command in the race has started to attract some bigger donors as well.

An Iowa Poll late last month showed Haley and DeSantis tied at 16%, with Trump at 43%.

Emily Seidel, a senior adviser to AFP, told reporters on a conference call that the group made the decision it did, in part, because “the top of the ticket sets the tone” for other Republican races lower on the ballot.

Seidel declined to say how much the group plans to spend on advertising in its effort to boost Haley, but stressed it will also be deploying its volunteers to knock on doors and mobilize voters. “We are confident that we are going to have the resources we need,” she said.

Asked whether the support is coming too late for Haley, Seidel dismissed the suggestion.

“We’ve done a significant amount of polling that shows that voters in the early states are just now starting to pay attention,” she said. “So, this is the sweet spot. This is the moment for our grassroots to come in and start talking to them and educating them on Nikki Haley’s policy record and encouraging them to get involved and support her.”

Seidel suggested Haley is the strongest-performing Republican candidate in a hypothetical matchup against President Biden in a general election, something recent national and battleground state polls have indicated.

In a statement, Haley said she was grateful for the support from a network that counts “millions of grassroots members” nationwide.

“AFP Action’s members know that there is too much at stake in this election to sit on the sidelines,” she said. “This is a choice between freedom and socialism, individual liberty and big government, fiscal responsibility and spiraling debt. We have a country to save, and I’m grateful to have AFP Action by our side.”

—Alex Leary contributed to this article.

Write to John McCormick at