The most helpful moment to Democrats may have come as Mr. Mueller faced his first questions, from Representative Jerrold Nadler of New York, the chairman of the Judiciary Committee. It is a sequence that is likely to play out on television and in political ads for months to come.
“Director Mueller, the president has repeatedly claimed that your report found there was no obstruction and that it completely and totally exonerated him. But that is not what your report said, is it?” Mr. Nadler asked.
“Right, that is not what the report said,” Mr. Mueller replied.
The exchange went on in that fashion, with Mr. Mueller shooting down Mr. Trump’s claims.
Time and again, Mr. Mueller defied Democrats looking for a flashy new moment.
It did not take long for the routine to become predictable: Democrats asked a leading or politically damaging question, and Mr. Mueller demurred. The special counsel clearly laid down limits.
“The most important question I have for you is why? Why did the president of the United States want you fired?” asked Representative Ted Deutch, Democrat of Florida.
“I can’t answer that question,” Mr. Mueller replied. It became a frequent refrain.
He repeatedly declined Democrats’ invitations to read passages from his report, consciously depriving Democrats of potentially useful footage of him speaking aloud some of the most damaging material he uncovered.
When Representative Hakeem Jeffries, Democrat of New York, walked through an analysis suggesting that several episodes documented by Mr. Mueller met the criteria for obstruction of justice, the former special counsel tossed cold water his way.
“I don’t subscribe necessarily to the way you analyzed that,” Mr. Mueller said.
The Democrats’ challenge was visible in miniature when Representative Jackie Speier, a California Democrat on the Intelligence Committee, offered Mr. Mueller an open platform to tell the American people why they ought to care about his work. He essentially refused to step onto it.
“We spent substantial time ensuring the integrity of the report,” Mr. Mueller said.
“It is a signal, a flag to those of us who have responsibility to exercise that responsibility, not to let this kind of thing happen again.”
Republicans tried to sow doubts, but Mr. Mueller frustrated them too.
Republicans’ playbook with Mr. Mueller was clear: Trumpet prosecutorial conclusions beneficial to Mr. Trump while trying to sow doubt about the basic fairness of Mr. Mueller, the F.B.I. and his team. More often than not, they met a stiff arm from Mr. Mueller, but succeeded in roughing him up around the edges.
There were questions — sometimes tangled and obscure — about shadowy figures in the investigation, about the supposed bias of Mr. Mueller’s team of investigators, and about charging decisions. Representative John Ratcliffe, Republican of Texas, told Mr. Mueller that he had “inverted burden of proof” by detailing the president’s conduct without charging him with a crime.
“Respectfully, respectfully, you managed to violate every principle in the tradition,” Mr. Ratcliffe said. “I agree with the chairman this morning when he said Donald Trump is not above the law. He’s not, but he damned sure shouldn’t be below the law” either.
Representative Devin Nunes of California, the ranking Republican on the Intelligence Committee, opened with, “Welcome everyone to the last gasp of the Russia collusion conspiracy.”
But Republicans gained little ground in their effort to better understand why Mr. Mueller or the F.B.I. made the choices they did. The former special counsel swatted away questions on a salacious but unverified dossier of information on Mr. Trump used by the F.B.I., on the former British spy who compiled it, and on other aspects of the origins of the Russia investigation.
Representative Jim Jordan of Ohio asked elaborately why Mr. Mueller chose not to charge Joseph Mifsud, the London-based professor who told a Trump campaign adviser that the Russian government had obtained “dirt” on Hillary Clinton in the form of thousands of emails. Mr. Jordan’s voice rose.
Mr. Mueller answered, “I can’t get into it.”
Whither impeachment? Mueller did not help advocates much.
Liberals who support opening impeachment proceedings against Mr. Trump had hoped that testimony by the former special counsel would finally electrify their efforts. The early verdict suggests that did not happen.
Mr. Mueller himself clearly did not want to let the term escape his mouth, nor did he provide the kind of shocking new evidence or analysis that would have forced the issue. When Representative Mike Johnson, Republican of Louisiana, asserted that the special counsel’s report did not recommend or even discuss impeachment, the witness would not even nod along.
“I am not going to talk about that issue,” Mr. Mueller said.
The staunchest supporters of the impeachment effort pressed on anyway, and are likely to keep up pressure on party leaders.
But with a six-week August recess looming and the views of most Americans fixed on what is now a two-year-old story line, a lasting shift in public opinion appears unlikely.
Mr. Mueller appeared a little shaky at the witness table.
In his years as F.B.I. director, Mr. Mueller was never known as a loquacious witness, but his performance on Wednesday frequently turned heads and prompted cringes from lawmakers looking on.
He stumbled over his words, asked lawmakers again and again to repeat their questions after misunderstanding or seemingly not hearing them, and declined to engage in any extended discussions over the legal rationale of his work. On occasion, Mr. Mueller, 74, appeared to be unfamiliar with details of his own report and stumbled over a confusingly worded question about his own résumé. That may have robbed his testimony of some of the power that many had expected.
When Representative Greg Stanton, Democrat of Arizona, asked which president had first appointed him as a United States attorney, Mr. Mueller hesitated, guessing that it may have been President George Bush.
“According to my notes, it was President Ronald Reagan who had the honor to do so,” Mr. Stanton said.
Mr. Mueller may have been confused by Mr. Stanton’s question because he was only an acting United States attorney under Mr. Reagan; it was Mr. Bush who later appointed him to a permanent Justice Department post.
In any case, Mr. Mueller did not correct the congressman. “My mistake,” he said.
But other moments proved stronger, particularly during the afternoon hearing with the Intelligence Committee. Mr. Mueller appeared more at ease and more willingly strayed from his written report.
The Justice Department’s opinion that bars charging a president brought confusion.
Democrats thought they had struck gold during the day’s first hearing when Mr. Mueller seemingly told Representative Ted Lieu, Democrat of California, that he would have indicted Mr. Trump if not for Justice Department policies prohibiting a federal indictment against a sitting president.
“The reason, again, that you did not indict Donald Trump is because of the O.L.C. opinion stating that you cannot indict a sitting president, correct?” Mr. Lieu asked, referring to the Justice Department’s Office of Legal Counsel.
“That is correct,” Mr. Mueller said.
Mr. Mueller’s statement directly contradicted what he wrote in his report and could have been damaging to Mr. Trump, implying that he was a criminal in all but name.
But when Mr. Mueller delivered an opening statement before the Intelligence Committee a short while later, he backtracked. He did not agree with Mr. Lieu’s statement, he said, repeating a version closer to what his team put in their report: that the policy prevented them from even considering whether to charge Mr. Trump.
“We did not reach a determination as to whether the president committed a crime,” he said.
Mr. Mueller offered a defense of his investigation, belatedly.
Through months of withering attacks by Mr. Trump and his allies, Mr. Mueller’s response was unchanged and often frustrating to his allies: total silence. On Wednesday, he finally pushed back, albeit late and with a light touch
“It is not a witch hunt,” Mr. Mueller declared flatly, when asked by Mr. Schiff about a term Mr. Trump has lobbed his way hundreds of times since the investigation began.
He betrayed stronger hints of emotion when Representative Tom McClintock, Republican of California, said, “Having desperately tried and failed to make a legal case against the president, you made a political case instead.”
Mr. Mueller replied, “I don’t think you have reviewed a report that is as thorough, as fair, as consistent as the report that we have in front of us.”
He had just as starchy a retort ready for Republicans who accused him of filling his office with partisan Democrats who were out to tank Mr. Trump.
“We strove to hire those individuals that could do the job,” Mr. Mueller, a Republican, said. “I’ve been in this business for almost 25 years, and in those 25 years, I have not had occasion once to ask somebody about their political affiliation. It is not done. What I care about is the capability of the individual to do the job and do the job quickly and seriously and with integrity.”