Daniel S. Loeb, the hedge fund manager, was one of Barack Obama’s biggest backers in the 2008 presidential campaign.

A registered Democrat, Mr. Loeb has given and raised hundreds of thousands of dollars for Democrats. Less than a year ago, he was considered to be among the Wall Street elite still close enough to the White House to be invited to a speech in Lower Manhattan, where President Obama outlined the need for a financial regulatory overhaul.

So it came as quite a surprise on Friday, when Mr. Loeb sent a letter to his investors that sounded as if he were preparing to join Glenn Beck in Washington over the weekend.

“As every student of American history knows, this country’s core founding principles included nonpunitive taxation,...