Jeff Yass, Republican Megadonor, Scrambled to Stop Doug Mastriano in Pennsylvania Governor’s Race

Jeff Yass, a Pennsylvania-based financier and a major Republican donor, had a straightforward request for Bill McSwain, a trailing candidate in that state’s G.O.P. primary for governor.

During a phone call last Tuesday, Mr. Yass asked: Will you consider dropping out?

Mr. McSwain, a former U.S. attorney who has been fielding many such calls lately, said he would think about it. He had hung in thus far, and with a week to go, why quit now?

Mr. Yass urged Mr. McSwain to consider the risk that if he stayed in the race, his candidacy could split the vote in a way that would help a polarizing far-right Republican candidate and put Democrats on a sure path to victory in the fall.

The far-right candidate, Doug Mastriano, a conspiracy-theory-minded state senator, leads polls ahead of the Tuesday primary. Many Republican leaders and donors have worried that if Mr. Mastriano wins the nomination, he would saddle the Republican Party with a candidate who might easily be defeated by Josh Shapiro, the attorney general of Pennsylvania, who is running uncontested on the Democratic side of the race.

Mr. Yass is a prominent supporter of charter schools, which are a major plank of Mr. McSwain’s campaign. As a state lawmaker, Mr. Shapiro voted against tuition vouchers for private schools, but has spoken in favor of the principle of allowing parents to have choices about where to send their children. He favors increased funding for public schools.

“This is about the cause, not about you,” Mr. Yass said he told Mr. McSwain. “If you drop out, we have a better chance of beating” what he called “wealthy Democrats like Josh Shapiro.”

In the end, Mr. McSwain decided to stick with his long-shot candidacy. The ballots were printed long ago, and it is unlikely that many voters would be aware if Mr. McSwain left the race. What little advertising money remained left was already being spent on pro-McSwain ads.

Mr. McSwain did not respond to multiple requests for comment. The phone call was described by five people who had been briefed on its contents and insisted on anonymity to discuss private conversations.

On Saturday, former President Donald J. Trump endorsed Mr. Mastriano, roiling the fractious Republican Party establishment in Pennsylvania, which is alarmed by the former military officer’s rapid ascent. Party leaders have scrambled to coalesce around the candidate thought to be in second place behind Mr. Mastriano, former Representative Lou Barletta, but many fear it may be too late to stop Mr. Mastriano from winning the nomination.

On Sunday night, the Commonwealth Partners Chamber of Entrepreneurs, a powerful Pennsylvania organization that is the top financial backer of Mr. McSwain’s campaign, endorsed Mr. Barletta.

“It’s become abundantly clear in recent days that nominating Lou Barletta for governor is Republicans’ best chance to defeat Josh Shapiro in November,” Matt Brouillette, the pro-business group’s chief executive, said in a statement.

Mr. Brouillette urged Mr. McSwain and another candidate, Dave White, to withdraw from the primary “for the good of our commonwealth and our country.”

In an interview, Mr. Brouillette said that his organization had concluded that Mr. Mastriano would lose to Mr. Shapiro in November. “We’ve determined mathematically he can’t win because he won’t appeal to key swing voters, given some of his extreme behavior,” Mr. Brouillette said.

Mr. Mastriano “wouldn’t be able to govern” even if he won, Mr. Brouillette added, pointing to the candidate’s poisonous relationship with many in the Pennsylvania Republican Party as well as his role in helping organize and attend the protests leading up to the attack on the Capitol in 2021. “He wouldn’t get anything done.”

In a statement issued after the Commonwealth Partners’ endorsement of Mr. Barletta, the McSwain campaign said he planned to stay in the race.

Mr. Yass, an intensely private player in Republican politics who co-founded the Susquehanna International Group, an investment firm, is a leading donor to libertarian causes. He is also a major funder of the Club for Growth, the influential anti-tax group.