Mr. Comey, who was once so broadly admired that his appointment to the post in 2013 was confirmed by the Senate 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.
Mrs. Clinton and her opponent, Donald J. Trump, who has used incendiary rhetoric toward Mr. Comey, are practicing the politics of total war, where there can be no noncombatants or neutral actors. Supporters have to be rallied, and adversaries must be distinguished from allies.
“Everything now gets weaponized,” said David Axelrod, a former adviser to President Obama. In their response to Mr. Comey, he said, “the Clinton people saw a strong rebuttal against him as a way of galvanizing their own forces.”
That Mrs. Clinton would make the F.B.I. director a target of her campaign’s wrath is deeply troubling to some Republicans, including those who are no admirers of Mr. Trump and believe that she is compounding what they see as the Republican candidate’s irresponsible behavior.
Even more worrisome, they say, is that the attacks are eroding what little faith the public still has in government.
“What we’re seeing in this election, with Trump saying the election and everything else is rigged and now this attack by Hillary and her people on the F.B.I., is basically an attack on our form of a constitutional government,” said former Senator Judd Gregg, a New Hampshire Republican. “They’re undermining the core elements of what gives our government credibility.”
William S. Cohen, who was a secretary of defense under President Bill Clinton, said the hostile rhetoric suggested that the country is “slouching towards the mutual assured destruction of our political system.”
“A healthy skepticism of public officials and institutions is descending into a corrosive cynicism that’s unlikely to be reversed in the short term,” Mr. Cohen said. “A coarsening of language is a precursor to the coarsening of values.”
Mr. Axelrod said Mrs. Clinton had a “legitimate” criticism with Mr. Comey, and her advisers felt they had no choice but to exert pressure on the F.B.I. director in order to reframe a potentially damaging narrative just over a week before the election. In the minds of Mrs. Clinton’s aides, Mr. Comey effectively made himself fair game when he issued a vague three-paragraph letter and inserted himself and his agency into the final days of a high-stakes contest.
“By taking this highly unusual, unprecedented action this close to the election, he put himself in the middle of the campaign,” Jennifer Palmieri, Mrs. Clinton’s communications director, said of Mr. Comey. The campaign’s advisers also felt they had to act, she added, because they believed that Mr. Trump would distort the letter.
Yet the evolution of Democrats’ handling of Mr. Comey’s letter illustrates just how tempting, and rewarding, total-war politics can be.
When Mrs. Clinton first addressed the inquiry on Friday evening, hours after news of Mr. Comey’s letter emerged, she was firm but also careful to avoid anything that could be seen as a personal criticism of the director. “It’s imperative that the bureau explain this issue and question, whatever it is, without any delay,” she told reporters in Iowa.
On Saturday, though, Mrs. Clinton saw an opportunity. When she referred to “the F.B.I. director,” the Democratic audience responded with a chorus of boos for an official who was appointed by Mr. Obama.
“It’s pretty strange to put something like that out with such little information right before an election,” Mrs. Clinton told supporters in Daytona Beach, Fla., winning applause and affirmation. “In fact, it’s not just strange; it’s unprecedented, and it is deeply troubling.”
And by Sunday, Mrs. Clinton’s top surrogates felt comfortable going even further, seeing that their supporters were roused by the 11th-hour story. Senator Tim Kaine of Virginia, Mrs. Clinton’s running mate, said at a campaign stop in Michigan, “It has kind of revved up some enthusiasm — a little bit of righteous indignation and righteous anger.”
John D. Podesta, her campaign chairman, when asked on CNN about how WikiLeaks had obtained his hacked emails, used the opportunity to ridicule the F.B.I. director.
“Maybe Jim Comey, if he thinks it’s important, will come out and let us know in the next nine days,” Mr. Podesta said.
Later that day, Mr. Reid all but accused Mr. Comey of violating the Hatch Act, which bars government officials from engaging in any activity that can influence elections.
“Through your partisan actions, you may have broken the law,” Mr. Reid, a Nevada Democrat, wrote in a letter to Mr. Comey.
What started as a request for additional information had escalated into claims of criminality in less than 48 hours.
Such accusations are unfamiliar territory for Mr. Comey, a Republican. He was hailed by the political left and center when, as deputy attorney general to President George W. Bush, he staged a made-for-Hollywood intervention at the hospital bed of Attorney General John Ashcroft to prevent other administration officials from making Mr. Ashcroft approve aspects of the National Security Agency’s domestic surveillance program.
And Mr. Comey was, of course, respected enough across party lines to be tapped by Mr. Obama for his current post.
“I’ve found him to be a straight guy,” Vice President Joseph R. Biden Jr. said of Mr. Comey on Friday shortly after the letter was disclosed.
Mr. Comey himself remained silent over the weekend. But by doing so, and by not disclosing further information about what new material his agents may have discovered, he has only fueled speculation.
“It is a shame how contentious everything now is, this state of our public discourse,” said Donald B. Ayer, a deputy attorney general under President George Bush. “But he broke the most fundamental rule as a prosecutor: You either put up or shut up.”
By deciding to effectively declare war on Mr. Comey — first for not bringing charges against Mrs. Clinton and then for showing renewed interest in the case — the two candidates have made him, and perhaps the bureau he leads, pay a price. After all, Mr. Comey may soon work for one of them.
“One more institution gets dragged into this partisan morass,” Mr. Axelrod said. “In the short term, it’s two candidates trying to win an election, but in the long term someone has to govern, and it becomes more difficult when everything is seen as an extension of politics.”