Tester, seen here in the Capitol in 2018, has leaned into his rural roots as he tries to hold onto his seat.

When his opponent, businessman and former Navy SEAL Tim Sheehy, won the Republican primary for Montana’s Senate seat in June, Sen. Jon Tester took to MSNBC to decry the money flowing in from out of state to support his opponent. “They’re trying to buy Montana, they’re trying to change it into a playground for the rich,” he said. “There’s going to be a lot of money put into this race trying to make me something that I’m not.”

Tim Sheehy does have plenty of billionaire backers who have cut checks to groups supporting him—and that’s not even counting former president and renewed Forbes 400 member Donald Trump, who has endorsed and campaigned with Sheehy (though hasn’t contributed a nickel). Montana’s deep-red partisanship on the presidential level makes Tester the most vulnerable Senate Democrat trying to keep his seat this year. But Tester, a farmer and three-term Senator, has plenty of billionaires and big money behind him, too.

Forbes examined Federal Election Commission records for every group that has opposed Sheehy or backed Tester, and found that every one of Tester’s top ten billionaire backers gave to Senate Majority PAC, a group that spends in or directs funds to Senate races across the country. (The group did not reply to a request for comment.) That approach stands in contrast to Sheehy’s conservative megadonors, who have given to a variety of different groups.

It’s also harder to track all of Tester’s biggest supporters. The super PAC that has spent the most in........

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