While he has remained largely outside the public eye, Mr. Klarman surprised some of his friends and peers over the summer when he issued a statement after Mr. Trump criticized a judge over his Mexican heritage, saying he planned to support Mrs. Clinton: “His words and actions over the last several days are so shockingly unacceptable in our diverse and democratic society that it is simply unthinkable that Donald Trump could become our president.”

In his recent letter, he explained for the first time his decision to say something publicly. “Despite my preference to stay out of the media,” he wrote, “I’ve taken the view that each of us can be bystanders, or we can be upstanders. I choose upstander.”

From the letter, it is hard to divine exactly how Mr. Klarman is investing his fund’s money. His office declined to comment on the letter, which I obtained from a source. His fund currently has more than 30 percent of its funds in cash. He has lost money in only three of the past 34 years.

What investors say publicly and what they do in the markets can be different things. Mr. Buffett campaigned publicly against Mr. Trump, but he has nevertheless invested in the market since his election — about $12 billion, according to a recent disclosure. George Soros, who also actively campaigned against Mr. Trump, bet — wrongly so far — that the stock market would fall; he lost about $1 billion.

Most hedge funds have found themselves on the losing side of trades over the past several years, a point Mr. Klarman addressed in his letter. Noting that hedge fund returns have underperformed the indexes — he mentioned that hedge funds had returned only 23 percent from 2010 to 2015, compared with 108 percent for the Standard & Poor’s index — he blamed the influx of money into the industry.

“With any asset class, when substantial new money flows in, the returns go down,” Mr. Klarman wrote. “No surprise, then, that as money poured into hedge funds, overall returns have soured.”

He continued, “To many, hedge funds have come to seem like a failed product.”

The lousy performance among hedge funds and the potential for them to go out of business or consolidate, he suggests, may become an opportunity.