Democrats and Republicans jostled for political points on Monday as they pressed the Justice Department’s inspector general for the first time over his exhaustive examination of the F.B.I.’s handling of its investigation into Hillary Clinton’s use of a private email server.
But over two hours of testimony before the Senate Judiciary Committee, it became evident that there was little that the inspector general, Michael E. Horowitz, could say to sway either side in their long political fight over what occurred at the F.B.I. in the run-up to the 2016 election.
Republicans raised doubts about key conclusions of Mr. Horowitz’s report, saying that they believed political bias among bureau officials may have improperly shaped its decision not to recommend charges against Mrs. Clinton. Democrats insisted that whatever F.B.I. officials had intended, their actions only served to harm Mrs. Clinton’s candidacy and potentially cost her the election.
Mr. Horowitz’s 500-page report, released Thursday after a yearlong investigation, found ample evidence of personal political bias by top F.B.I. officials working on the Clinton investigative team and said it cast a cloud over the F.B.I. and its work. Mr. Horowitz’s report found that actions by James B. Comey, the former F.B.I. director, had likewise harmed the bureau. But Mr. Horowitz said he found no evidence that F.B.I. officials’ political views influenced prosecutorial decisions in the Clinton case that he reviewed, though he expressed deep concern about their apparent openness to the idea.
“We found the implication that senior F.B.I. employees would be willing to take official action to impact a presidential candidate’s electoral prospects to be deeply troubling and antithetical to the core values of the F.B.I. and the Justice Department,” Mr. Horowitz told senators.
Citing findings that two F.B.I. officials had exchanged text messages disparaging — and in one case pledging to “stop” — Donald J. Trump when he was a candidate, Republican senators said that it stretched credulity to believe that the team investigating Mrs. Clinton had stopped short of acting on their political convictions.
“We can’t just write it off,” said Senator Lindsey Graham, Republican of South Carolina. “I think there was a lot of bias that did affect an investigation that is to me almost impossible to explain using any standard that I grew up with as a prosecutor or even a defense attorney.”
Mr. Graham urged Mr. Horowitz to reconsider his findings.
Senator Charles E. Grassley, Republican of Iowa and committee’s chairman, warned that the Justice Department faces a “serious credibility problem” for apparently applying different standards to Mrs. Clinton and Mr. Trump.
“They see a story of kid-glove treatment for one side and bare-knuckle tactics for the other,” he said.
Under questioning from Mr. Grassley, Mr. Horowitz confirmed that his office was investigating whether Mr. Comey had acted inappropriately in another case, when he provided confidants outside the F.B.I. with copies of memos he wrote about his conversations with Mr. Trump.
“We received a referral on that from the F.B.I.,” Mr. Horowitz said. “We’ll issue a report when the matter is complete.”
Democrats zeroed in on Mr. Comey’s earlier actions: namely, his decision to break with longstanding policy to publicly discuss the Clinton case — the decision they said might have cost Mrs. Clinton the presidency, not helped her. And though they conceded that political statements by the investigative team were inappropriate, they said Mrs. Clinton’s election loss provided proof to support Mr. Horowitz’s conclusions.
“If the F.B.I. was trying to throw the election to Hillary Clinton, it couldn’t have done a worse job,” said Senator Patrick J. Leahy, Democrat of Vermont. “Every single misstep by the F.B.I. damaged Hillary Clinton and helped Donald Trump.”
As senators circled Mr. Comey, his successor as F.B.I. director, Christopher A. Wray, tried to show lawmakers that he intended to chart a different course.
Mr. Wray repeatedly defended the F.B.I. but acknowledged serious missteps. He said that it was not the job of the F.B.I. to make charging decisions, and he was critical of Mr. Comey’s decision to hold a news conference in July 2016 in which he declared that the F.B.I. would not recommend charges against Mrs. Clinton but harshly criticized her handling of sensitive government information.
“Mistakes made by those employees do not define our 37,000 men and women and the great work they do every day,” Mr. Wray said. “Nothing in this report impugns the integrity of our work force.”
Democrats demanded to know whether anyone at the F.B.I. had provided information during the campaign to Rudolph W. Giuliani, who appeared to hint at the time that he knew that agents had reopened the Clinton email investigation in October.
Two days after the election, Mr. Giuliani told Fox News, “I had expected this for the last — honestly, tell you the truth, I thought it was going to be about three, four weeks ago.” He added: “I did nothing to get it out. I had no role in it. Did I hear about it? You’re darn right I heard about it.”
Neither Mr. Wray nor Mr. Horowitz would comment on whether there was an investigation into whether anybody at the F.B.I. provided information to Mr. Giuliani, who is now a lawyer for Mr. Trump in the special counsel investigation.
Mr. Wray also took a swipe at Mr. Comey for previously disclosing the F.B.I. had been investigating any leaks to Mr. Giuliani. “There are a number of things I probably would have done differently,” Mr. Wray said with a wry smile and raised eyebrow.
Mr. Wray also promised to crack down on leaks to the press, and he said he had created a unit to investigate unauthorized disclosures of classified information.
“Leaks are wrong,” he said. “And we need to be tough on them.”
Mr. Wray also continued to express confidence in the special counsel leading the Justice Department’s investigation into Russian election interference and Mr. Trump’s campaign, Robert S. Mueller III.
“I do not believe that Special Counsel Mueller is on a witch hunt,” he said, later adding, “I believe Special Counsel Mueller is conducting an important investigation.”