WASHINGTON — Robert Mueller, the famously tight-lipped former special counsel, began testifying before Congress on Wednesday at the start of a pair of hearings that Democrats hope could change the trajectory of Donald Trump’s presidency, even if it reveals little new information.

Mueller, who has spoken publicly only once since he was appointed to run the government’s investigation of Russian efforts to sway the election that put Trump in office, is appearing before two House committees in a back-to-back testimony. He faces questions from Democrats and Republicans, both factions eager to score points from his remarks even as many lawmakers have already made up their minds about whatever it is he might have to say.

Democrats are betting that the spectacle of Mueller’s public appearance will carry far more weight than a 448-page document his investigation produced. They’re hoping that words from Mueller himself would be pivotal and would make the case that the president’s conduct should be punishable by impeachment or a 2020 defeat. 

Republicans, who have long questioned the integrity and genesis of the Russia inquiry that they say exonerated the president, will likely use the rare public appearance to highlight the lack of charges against Trump and the perceived political motivation behind the probe. The president and his allies have accused Democrats of trying to redo the investigation by staging a belated spectacle. 

Rep. Jerry Nadler, chairman of the House Judiciary Committee, began Wednesday’s hearing with a summary of Mueller’s years in public service as a Marine officer who was awarded a Purple Heart and as director of the FBI.

“Director Mueller, we have a responsibility to address the evidence you’ve uncovered,” Rep. Jerry Nadler, D-N.Y., chairman of the House Judiciary Committee, said in a prepared opening statement. “You recognized as much when you said, ‘the Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing.'”

Nadler said the hearing would highlight episodes in which Trump sought to thwart Mueller’s investigation. “Any other person who acted this way would have been charged with a crime. And in this nation, not even the president is above the law.”

More: Former special counsel Robert Mueller’s testimony, like his report, promises an ink-blot test for partisans

More: Democrats betting on Robert Mueller’s public testimony to make the case his report so far has not

Even before Mueller’s testimony began, Trump began issuing a series of pointed – and now familiar – critiques of the former special counsel and the investigation he ran. “Why didn’t Robert Mueller investigate the investigators?” Trump wrote in an early morning tweet, repeating unproven claims that Mueller had conflicts of interest and claiming that he had been the victim of “The Greatest Witch Hunt” in history.

In a letter made public on Monday, Associate Deputy Attorney General Bradley Weinsheimer told Mueller that “any testimony must remain within the boundaries” of his report, a redacted version of which was made public in April. Weinsheimer said information such as presidential communications, discussions about investigative steps and decisions made during the investigation can’t be disclosed.

The department’s longstanding policy also prohibits publicly discussing the conduct of “uncharged third-parties,” Weinsheimer said. 

Rep. Adam Schiff, chairman of the House Intelligence Committee, told Mueller in his own letter late Tuesday that the Justice Department’s restrictions “will have no bearing” on what Mueller can and can’t say.

Mueller’s spokesman, Jim Popkin, reaffirmed that the former FBI director will not veer from his report’s findings. Mueller will deliver an opening statement to the House Judiciary and Intelligence committees that is expected to closely track his first public remarks in May, when he defended the investigation, Popkin said Monday.

Mueller spent two years investigating Russian interference in the 2016 presidential election and whether Trump obstructed the inquiry that consumed Washington and the first half of his presidency. Trump and his allies spent nearly as much time questioning the basis of the investigation, deriding it as a witch hunt undertaken by a conflicted special counsel and politically biased investigators, some of whom the president has accused of spying and treason. 

More: Robert Mueller, in first public remarks, says charging Trump was ‘not an option we could consider’

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Mueller’s testimony comes about three months after the release of his exhaustive report. The investigation revealed a systematic effort by Russia to sway the election in Trump’s favor and a campaign that embraced the assistance but did not conspire with the Kremlin. The report also said Trump repeatedly tried to impede the Russia investigation, though Attorney General William Barr and former deputy attorney general Rod Rosenstein determined that the evidence does not establish Trump had committed a crime.

During the brief public appearance in May, Mueller did not clear Trump of criminal wrongdoing but said charges were “not an option” because of Justice Department policy of not indicting a sitting president.

“If we had confidence that the president clearly did not commit a crime, we would have said that,” Mueller said. 

Mueller also said that allegations of “multiple” and “systematic” Russian interference in U.S. elections deserve “the attention of every American.”

The House Judiciary Committee will first question Mueller for three hours before the House Intelligence Committee gets its turn.

In his letter to Mueller, Weinsheimer underscored the forced nature of the testimony.

“As the attorney general has repeatedly stated, the decision to testify before Congress is yours to make in this case, but the department agrees with your stated position that your testimony should be unnecessary under the circumstances,” Weinsheimer wrote. “The department generally does not permit prosecutors such as you to appear and testify before Congress regarding their investigative and prosecutorial activity.”

Contributing: Bart Jansen

More on Robert Mueller and the Russia investigation:

Trump’s aides were eager to take Russian dirt on Clinton. But it wasn’t a conspiracy, Mueller report said

Trump repeatedly tried to impede the Russia probe, Mueller report said. Was it obstruction?

Trump took steps to fire Mueller, stop probe after campaign welcomed Russian dirt on Clinton, Mueller report says

Spying, treason and politics: President Trump ups the stakes in Russia probe battle despite scant evidence