A Republican mega-donor, Peter Thiel, is washing his hands of his handpicked Senate candidates as they enter the final sprint of campaign season.

Mr. Thiel opened the primary season with $10 million and $15 million wagers on venture capitalist Blake Masters in Arizona and J.D. Vance in Ohio, two friends and former associates of the Paypal co-founder. Both managed to secure the Republican nominations, but they’re now struggling to raise cash for the final legs of their races.

Mr. Thiel’s checkbook is missing in action, and Messrs. Masters and Vance have only $1.6 million and $628,000 on hand, respectively, according to the latest Federal Elections Commission filings. Mr. Thiel has and is, however, hosting fundraisers for both candidates, so that other people can contribute.

For comparison, their opponents, Senator Kelly in Arizona and Congressman Tim Ryan in Ohio, have $25 million and $3.5 million, respectively.

To help fill the funding gap, the Senate Leadership Fund, a political action committee aligned closely with the Senate minority leader, Mitch McConnell, stepped in. It has bought some $14.4 million worth of ads in Arizona and $28 million worth in Ohio, and though the bulk of the ads are yet to air, it’s clear that they will have some heavy lifting to do.

Democrats, if the polls are to be believed, appear to be winning in both races, though those polls may shift before Election Day. In Arizona, Mr. Kelly enjoys a healthy lead of 7.8 points, according to FiveThirtyEight’s average. In Ohio, Mr. Ryan enjoys a more modest lead of 1.9 points.

The polls, however, may reflect an outdated version of the electorate. Women, fired up perhaps by the Dobbs ruling from the Supreme Court, have been registering to vote in unusually high numbers in both states, which could hurt Republicans’ chances in both states.

Mr. Thiel’s conspicuous absence comes at an inopportune time for Republicans, with party leadership quibbling over funding and dealing with organizational problems.

The chairman of the National Senatorial Committee, Senator Scott of Florida, has spent the vast majority of the $181 million it raised, leaving the organization with only $23.2 million to spend across all the party’s candidates.

The buck has been passed to the Senate Leadership Fund, and though Mr. McConnell has agreed to pay up in both states, the investments have an opportunity cost — particularly in Ohio, a state that should have been safe for the GOP and where the committee has invested more than any other state.

The Sun caught up with a political scientist at Ohio University, Benjamin Bates, who said that Mr. Thiel’s influence — coupled with President Trump’s endorsement — helped snub the Republican Party’s preferred candidate, Josh Mandel, the Ohio treasurer.

“After the primary, Donald Trump and Peter Thiel seem to have disappeared until just recently,” he tells the Sun. “He’s a venture capitalist, so people say, ‘Maybe he’s pulled out of Ohio because he doesn’t see the big payoff that he does in other races.’”

By Mr. Bates’s calculation, Mr. McConnell and others saddled with footing the bill for Mr. Vance’s campaign can’t be happy with their influence in the race.

“Thiel is a disruptor and an unwelcome disruptor in the minds of the people that have to pay for the candidates that Thiel is no longer supporting,” Mr. Bates said.

Mr. Bates said Mr. Vance’s campaign is largely on-message — “when they’re present” — focusing chiefly on economic concerns. However, much like his one-time mentor Mr. Thiel, Mr. Vance also is conspicuously absent from the public eye.

As a Cincinnati talk radio host, Bill Cunningham, said on CNN, supporters and members of “the Republican Party are saying, ‘Where’s J.D.?’”