WASHINGTON — In July, after the F.B.I. director, James B. Comey, declined to bring charges against Hillary Clinton for her handling of classified emails, leading Democrats hailed his leadership.

“This is a great man,” said Nancy Pelosi, the House minority leader. “We are very privileged in our country to have him be the director of the F.B.I.” No one, added Harry Reid, the Senate minority leader, “can question the integrity” of Mr. Comey.

But on Friday, after Mr. Comey revealed that the F.B.I. would review newly discovered emails potentially related to the case, Democrats changed their tune.

Mr. Reid all but accused him of criminality, writing to Mr. Comey, “You may have broken the law.”

Mr. Comey, who was once so broadly admired that the Senate confirmed his appointment in 2013 by a vote of 93 to 1, has emerged as the most vivid example of how difficult it is for institutions to remain insulated from partisan combat in this hyperpolarized era.