Whatever shortcomings Mrs. Clinton may have as a candidate, Saturday’s coordinated effort showed that the political organization that she, her husband and her allies had built over decades remained potent and would not let what seemed like victory erode easily. By midday, Mr. Comey, a Republican appointed by President Obama and confirmed nearly unanimously by the Senate, found himself in its cross hairs.

Encouraged by Mrs. Clinton’s senior aides to reframe the story and make it about Mr. Comey’s actions, liberal groups such as the Congressional Black Caucus demanded that he release more information. Other surrogates were emailed talking points prodding them to deem it “extraordinary that 11 days before the election a letter like this — with so few details — would be sent to 8 Republican committee chairmen.” (Ranking Democrats on the committees also received copies.)

Mr. Comey has not publicly commented on the investigation, other than with the letter saying that more emails were being examined. He also wrote an email to F.B.I. employees explaining that he felt he had to inform Congress even though the agency did not yet know “the significance of this newly discovered collection of emails.”

With Mrs. Clinton leading Donald J. Trump in nearly every battleground state, Clinton advisers were emphatic that they would not be thrown off stride. They said they would not change any political strategy, television advertising or campaign travel plans.

For months, the F.B.I. had investigated whether Mrs. Clinton had broken any laws by using a private email server while she was secretary of state. This past summer, Mr. Comey said that Mrs. Clinton had been “extremely careless” by allowing sensitive information to be discussed outside secure government servers, but that the agency had concluded that Mrs. Clinton had not committed a crime. The investigation was closed.

But on Friday, Mr. Comey notified Congress that the agency had discovered emails, possibly relevant to the investigation, that belonged to Mrs. Clinton’s top aide, Huma Abedin. The emails were discovered on the computer of Ms. Abedin’s estranged husband, Anthony D. Weiner, during a separate investigation into allegations that he had exchanged sexually explicit messages with a teenager.