“May I finish?” the witness asked. “May I please finish?”

The appeal was made in June 2013 by Robert S. Mueller III, then in the twilight of his term as the FBI director. Bent over the witness table in a House Judiciary Committee hearing room, he put up his left hand in protest.

The top law enforcement official asked Rep. Jim Jordan, the Republican of Ohio, to stop interrupting him. The lawmaker fired back: “I’m asking basic questions about the investigation.”

At issue six years ago in the oversight hearing were allegations that the IRS had improperly targeted conservative groups for scrutiny.

The topic when Mueller appears before Congress next month will be an even more explosive one. The former special counsel is scheduled to testify on July 17 before the House Judiciary and Intelligence committees about his investigation into Russian interference in the 2016 election and possible obstruction of justice by President Trump.

The president offered only an abbreviated response to the announcement Tuesday evening from the committee’s two Democratic chairmen. “Presidential Harassment!” Trump wrote on Twitter.

But the president’s loyalists in Congress — Jordan among them — will probably have much more to say to Mueller, whose 22-month investigation concluded with a report indicating that prosecutors did not decide whether Trump had engaged in criminal behavior because of Justice Department policy preventing the indictment of a sitting president.

The Fact Checker's road map to special counsel Robert S. Mueller III's report and his decision not to bring charges against Trump. (The Washington Post)

Stepping down from his role in May, Mueller spoke briefly about his team’s findings, saying that if prosecutors “had confidence that the president clearly did not commit a crime, we would have said so.” The Constitution, he said, “requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing.”

“Any testimony from this office would not go beyond our report,” he said at the time.

That vow is unlikely to stop lawmakers —— Republicans and Democrats alike — from trying to draw out the flinty former special counsel.

Rep. Douglas A. Collins of Georgia, the top Republican on the House Judiciary Committee, said in a statement that he hoped Mueller’s testimony would “bring to House Democrats the closure that the rest of America has enjoyed for months.”

Asked on Fox News Tuesday night what questions he would put to Mueller, Rep. Matt Gaetz, the Florida Republican who sits on the Judiciary Committee, did not list any specific questions but likened Mueller’s team to Chernobyl, the 1986 nuclear disaster in Soviet Ukraine.

For his part, the Democratic chairman of the Judiciary Committee, Rep. Jerrold Nadler of New York, suggested on CNN that Democrats would aim to find out where Mueller disagreed with the framing of his report following its summary by Trump’s attorney general, William P. Barr.

Insight into their possible tactics in questioning Mueller, and into the way he might respond, lies in his previous appearances before Congress. They show that adversarial committee members have been ready to go after the top law enforcement official, pressing him for details about investigations and faulting him for being unable or unwilling to answer. So, too, the encounters show that Mueller has been an assertive witness, unafraid to return fire and accuse lawmakers of making erroneous claims.

“Your facts are not altogether — ” Mueller told Rep. Louie Gohmert, the Texas Republican, during the same 2013 oversight hearing in which he clashed with Jordan over the IRS. Both men still sit on the Judiciary Committee.

As his microphone was cut off, the witness appeared to say that Gohmert’s facts were not “well-founded.”

The lawmaker was asking why the FBI had not canvassed Boston mosques before the detonation of two homemade pressure cooker bombs that killed three people and injured hundreds more near the finish line of the Boston Marathon in April 2013.

“May I finish my —” Mueller continued.

But Gohmert interjected: “Sir, if you’re going to call me a liar, you need to point out specifically where any facts are wrong.”

The exchange ended in an impasse, as Mueller insisted he had already answered the lawmaker’s question, and Gohmert insisted he had not.

Mueller is well-practiced at answering questions before Congress and not answering others.

Since President George W. Bush nominated him to head the FBI in the summer of 2001, Mueller has made dozens of appearances before Congress. The man whose voice Americans hardly know has spent hours speaking into microphones at congressional hearings, preserved by C-SPAN.

Mueller has answered questions on a range of hot-button topics, in front of both friendly and hostile audiences. Beginning his tenure at the FBI just days before the 9/11 attacks, Mueller was called to testify before a joint House-Senate panel on intelligence gathering. In the years since, he has regularly appeared to defend budget requests and to comply with congressional oversight. He has been asked to weigh in on momentous questions, from the Patriot Act to Russian espionage.

Perhaps most pertinent to the topic of the July hearings is a line of questioning pursued at Mueller’s confirmation hearing in 2001.

The questioner was none other than Jeff Sessions, the former senator from Alabama who was forced out as Trump’s attorney general in November. His resignation followed months of escalating attacks from the president, who resented the Cabinet official for recusing himself from handling the Russia investigation.

The confirmation hearing unfolded in the wake of the impeachment of President Bill Clinton, who was acquitted by the Senate in February 1999. The legal saga, fresh in the memory of Republican senators, prompted them to ask Mueller how he would manage an inquiry implicating the president.

The concern emphasized by the GOP lawmakers was that an FBI director — and law enforcement more generally — might be swayed by political pressure exerted by an attorney general. The worry finds a parallel today in criticism of Barr’s handling of Mueller’s conclusions.

“Under those circumstances, I hope that you will keep your options open, because you have a 10-year appointment,” Sessions told the nominee. “That is for a reason, so that if something serious occurs, and there has been a threat to the orderly operation of justice, that you would use that independence for a good reason.”

It wasn’t a question. But Mueller asked: “May I respond to that, Senator?”

"Please,” Sessions told him.

Mueller allowed that there could be circumstances in which he would “feel it necessary to circumvent the ordinary course of proceedings,” sidestepping the authority of the attorney general. He pointed to a situation “where one believes that political pressure is being brought to bear on the investigative process.” He said he might look “somewhere else in the executive, beyond the attorney general,” or else possibly disclose his misgivings to Congress.

“But I would look and explore every option if I believed that the FBI was being pressured for political reasons,” he said. “And if that were the situation as described here, I would explore other alternatives or a variety of alternatives in order to make certain that justice was done.”

Mike DeBonis contributed to this report.

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