Interviewed Friday on “Real Time with Bill Maher” on HBO, Sanders was asked if the furor over Clinton’s use of a private email server as secretary of state had become large enough for the Vermont senator to reconsider his refusal to engage Clinton on the issue.

“It has,” he said. “But this is what I also think: There is an enormous frustration on the part of the American people.”

The State Department’s inspector general found in a report released this past week that the email setup violated department rules, that Clinton never sought permission for it, and that the proposal would have been rejected if she had. The report handed Clinton’s Republican opponents a fresh line of attack — and Sanders, too, if he chose to take it. Clinton’s competitor for the Democratic presidential nomination won praise at a candidates’ debate on October when he said, “Enough of the emails. Let’s talk about the real issues facing America.” At the time, his campaign used the comments in a fund-raising email.