Mr. Reid was preparing to send a letter to Mr. Comey on Sunday saying he appears to be aiding one political party over another. He said that may violate the Hatch Act, which bars government officials from using their position to influence an election.

“I am writing to inform you that my office has determined that these actions may violate the Hatch Act,” Mr. Reid wrote, according to a draft of the letter provided to The Wall Street Journal. “Through your partisan actions, you may have broken the law.”

A spokesman for the Federal Bureau of Investigation didn’t immediately respond to a request for comment.

The opinion from Mr. Reid, a supporter of Mrs. Clinton, carries more political weight than legal significance and comes as Democrats aim to circle the wagons around their presidential nominee in the final days of the campaign.