HARRISBURG — What does a conservative upstart challenging a top House Republican in York County have in common with a scion of a Democratic political family facing a progressive primary in Philadelphia?

Despite differing priorities and positions, both candidates’ campaigns are beneficiaries of the political largess of billionaire Jeff Yass, a Montgomery County investor and Pennsylvania’s richest denizen. A billionaire who turned gambling winnings into one of the largest stock trading companies in the U.S., Mr. Yass is a registered Libertarian, according to Pennsylvania Department of State records.

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Mr. Yass’ allies, including many who spend or have sought his dollars, argue he is a single-issue donor who backs candidates who support alternatives to public schools.

“It’s no secret,” longtime conservative activist Matt Brouillette said. “Where Jeff and [his wife] Janine invest their philanthropy and political dollars, they are passionate about getting kids to better schools.”

But critics from various political backgrounds argued Mr. Yass’ spending prioritizes peddling influence or inserting his ideology into both major parties, whether the issue is privatizing public schools or cutting taxes.