The 61-year-old, who has $30 billion under management at Baupost Group and is estimated by Forbes to be worth $1.5 billion, is compared regularly with Warren Buffett. Paul Singer, one of the most influential hedge fund managers in the country, told me that if he was going to invest in any fund other than the one he himself manages, “the only one would be Seth’s.” Used copies of a book about investing that Mr. Klarman wrote in 1991 go for more than $650 on Amazon.

“I’m kind of a loner,” Mr. Klarman said last week in his 17th-floor office close to the Boston Common. “One of the reasons I’m willing to come out of my shell and talk to you is because I think democracy is at stake. And maybe I’ll be able to convince some other people of that. And get them to support Democrats in 2018.”

Not two years ago, Mr. Klarman, a registered independent, was the biggest donor to the Republican Party in New England. According to The Boston Globe, during the Obama administration, Mr. Klarman gave more than $7 million to the party. If you look at his Federal Election Commission filings for 2016, you will find $100,000 to the Hillary Action Fund, but mostly a long list of donations to names like Marco Rubio, Paul Ryan, Chris Christie and the Republican National Committee.

No longer. Among the many things Donald Trump has upended, one has been Mr. Klarman’s political giving. The denunciation of the president and his party in papers like this one has done nothing to change their behavior. He hopes money will.

The F.E.C. filings that will come out on Sept. 20 will show that Mr. Klarman is now giving almost exclusively Democrats — and donating far more money than he ever has.

Mr. Klarman shared an early copy with me. So far this cycle he’s ponied up $4.9 million. He has given to 150 or so candidates, including Joe Kennedy, Conor Lamb, Beto O’Rourke and Kirsten Gillibrand, as well as dozens of lower-profile candidates. Liberal super PACs aimed at helping the Democrats take back the Senate and the House received $1 million and $2 million, respectively. It’s quite a list for someone who says, “I’m not a Democrat.”

For Mr. Klarman, the logic is plain: “We need to turn the House and Senate as a check on Donald Trump and his runaway presidency.”

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He referred to what’s going on in Washington as an “emergency” multiple times and suggested that the question patriots should ask themselves about the midterms is: What’s the price tag for democracy? If you knew your donation would give your grandchildren a better chance of growing up in one, how much would you contribute? “It seems like the answer should be a huge amount,” he said.

“If one looks at my past giving, one might think that several million, five or six million, might be a lot in a cycle.” This time, he said, “I’m stretching far beyond what I usually do.”

How far? “By the election I think I’ll have spent between $18 and $20 million.”