“Reading this report clearly makes me sick,” said Donna Brazile, who was chair of the Democratic National Committee during the 2016 general election. “It confirms what we all believed at the time.”

The report, Ms. Brazile said, strengthened her view “that 2016 will always be an election where there’s an asterisk.”

Randi Weingarten, the president of the American Federation of Teachers and a longtime Clinton ally, said the report showed there had been a “double standard” in the election, whereby the F.B.I. revealed information casting Mrs. Clinton in an unflattering light while concealing investigations into Mr. Trump.

“It’s disappointing and infuriating,” Ms. Weingarten said. “There is a reason for these norms of not commenting, knowing full well that comments can sway public opinion.”