When Jamie Dimon, the chief executive of JPMorgan Chase, urged Democratic donors last week to rally behind Nikki Haley to provide Republican voters an alternative to former President Donald J. Trump, it seemed a far-fetched plea.

But at least one of the Democratic Party’s biggest financiers has already done exactly that.

Reid Hoffman, the billionaire co-founder of LinkedIn and a major Democratic donor, recently gave $250,000 to a super PAC supporting Ms. Haley, the former South Carolina governor who has gained momentum in recent weeks in the 2024 Republican primary race. The donation, which has not been previously reported, was confirmed by Dmitri Mehlhorn, a political adviser to Mr. Hoffman.

The pro-Haley super PAC, SFA Fund Inc., was asked specifically by Mr. Hoffman’s political team if it would take money from Mr. Hoffman, given that he is a Democrat who actively supports President Biden, Mr. Mehlhorn said. The super PAC, he added, said yes.

The pro-Haley super PAC did not immediately respond to a request for comment.

SFA Fund Inc. has been one of the biggest players in the 2024 Republican primary race, spending more than $33 million on advertising and other expenses. Its biggest contributors in the first half of the year were Jan Koum, a co-founder of WhatsApp, who gave $5 million, and the venture capitalist Tim Draper, who gave $1.25 million.

Mr. Hoffman has financially backed an array of anti-Trump candidates and causes. He helped fund the lawsuit by E. Jean Carroll, the writer who sued Mr. Trump for rape and defamation. In May, a Manhattan jury found Mr. Trump liable for sexually abusing and defaming her.

The extra money will help Ms. Haley’s super PAC buy more television ads as she battles to separate herself from Gov. Ron DeSantis of Florida and close the gap in polling with Mr. Trump. But such cross-party giving is rare and could also risk a backlash. Even before Mr. Hoffman’s donation was made public, Mr. Trump had seized on coverage of Mr. Dimon’s comments urging Democrats to back Ms. Haley and used it in fund-raising messages.

“While globalist special interest donors from both parties forge an unholy alliance to beat us, I’m calling on our grass roots donors like YOU to fight back,” Mr. Trump wrote in a fund-raising email on Monday.

Ms. Haley herself has explicitly made appeals for Democratic backing.

“Anybody that wants to come support the cause, whether they’re Republican, independent or Democrat, we’re going to take them,” she said on Fox News last week. “And that’s the way the Republican Party should look at it is, this is a story about addition, not about getting people and pushing people away.”

After The New York Times wrote about the donation, the DeSantis campaign criticized Ms. Haley for her unlikely supporter. “It makes perfect sense that liberal Democrat billionaires would support Nikki Haley’s bid for the White House,” Bryan Griffin, a DeSantis spokesman, said in a statement, “because she is a liberal.”

A former Trump adviser, Stephen K. Bannon, also attacked Ms. Haley after the donation was reported, calling her a “phony” on his streaming program and Mr. Hoffman a “Trump hater of just epic proportions.”

Mr. Hoffman has been involved in the Republican primary in other ways, too. Mr. Mehlhorn was in conversations this year on behalf of Mr. Hoffman with a super PAC backing former Gov. Chris Christie of New Jersey, though Mr. Hoffman did not give directly to that group.

But a nonprofit group with close financial ties to Mr. Hoffman, Defending Democracy Together, did publicly report making a $500,000 donation in June to the pro-Christie super PAC, Tell It Like It Is PAC. Mr. Mehlhorn said that Mr. Hoffman had given even more to Defending Democracy Together since then.

Colin Reed, a spokesman for the Christie super PAC, said the group would have happily taken Mr. Hoffman’s money directly. “We’re grateful of the support of anyone who wants to support the governor’s campaign,” Mr. Reed said.

Mr. Hoffman, who co-hosted a fund-raiser for Mr. Biden in California this year, has been uniquely motivated as a donor in opposition to Mr. Trump. He donated in 2022 to Liz Cheney, who had emerged as one of the Republican Party’s most vocal anti-Trump leaders.

In the 2022 election cycle, Mr. Hoffman also gave $3.35 million to the Republican Accountability Project, which opposes Mr. Trump’s takeover of the party.

In a post on LinkedIn eight months ago, Mr. Hoffman said he had helped pay for Ms. Carroll’s case as a counterweight to “Donald Trump’s scorched-earth legal methods.”

“Supporting women fighting for progress and justice in philanthropy, politics, and business has been a longstanding priority of mine, as is supporting America against the threat of Trump,” he wrote.