Senator Bernie Sanders has long said he has no interest in making the saga of Hillary Clinton’s emails a campaign issue, but in the moments on Tuesday after the F.B.I. said that she would not face charges, many of his most fervent supporters seethed with disappointment.

Since Mrs. Clinton secured enough delegates to secure the Democratic presidential nomination last month, Mr. Sanders’s campaign has become a shadow operation intent on influencing the party’s platform at the convention July 25-28 in Philadelphia. Although Mr. Sanders has acknowledged that his chances of being the nominee have faded, some of his backers had felt that the investigation represented a small glimmer of hope. The inquiry’s conclusion all but extinguished that.

“A lot of people are disappointed because they saw this as their way in,” said Steve Todd, a delegate from Pennsylvania who supports Mr. Sanders. “If she were to be indicted, there’s no one else who could start from scratch and jump in — it would be ours for the losing.”

But the lack of any charges reignited the infighting that is lingering within the Democratic Party, with some people who voted for Mr. Sanders in the primaries viewing the outcome as another example of favoritism for the political elites.