Supporters of Hillary Clinton at her election night event in Manhattan in November 2016.

Democrats have yearned for a moment of political exoneration ever since Hillary Clinton’s defeat in 2016. They have looked to Russian interference in the campaign, claims of bias in the media and allegations of Republican lawbreaking to explain an upset that few in the party foresaw.

Perhaps most of all, Democrats have vented indignation at the F.B.I. and its former director, James B. Comey, for reviving the issue of Hillary Clinton’s private email server in the last days of the race.

On Thursday, Clinton supporters won a powerful kind of validation from the unlikeliest source: President Trump’s Department of Justice.

The inspector general’s report criticizing Mr. Comey for his flamboyant handling of the Clinton investigation sent an angry thrill through the ranks of Democrats and Mrs. Clinton’s allies. Michael E. Horowitz, an investigator not appointed by Mr. Trump, concluded that Mr. Comey had twice breached the bureau’s traditional discretion: first by holding a July news conference to announce he would not charge Mrs. Clinton with mishandling classified information, and then later sending a letter to Congress disclosing that the agents were scrutinizing new evidence in the matter.

In many respects, those findings mirrored Democrats’ own assessments of Mr. Comey — save for the omission of certain four-letter words.

But if the report appeared to validate their grievances against Mr. Comey, it offered scant relief to Clinton loyalists. For some of them, it intensified the agony of Mr. Trump’s surprise win — cementing Democratic suspicions about the fairness of his election, but leaving them without recourse to address them.

Hillary Clinton during a news conference following her loss to Donald J. Trump in the 2016 presidential election.

“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.”

While Ms. Weingarten called the inspector general’s report a “service to the country,” she said it brought no solace on a personal level. “There’s no sense of…