The eyes of America will be trained on Capitol Hill on Wednesday, as Robert Mueller testifies before two House committees about his report on Russian election interference, links between the Trump campaign and Moscow and potential obstruction of justice by the president.

On Sunday, the chairman of the judiciary committee indicated the stakes when he said the 448-page report contained “very substantial evidence that the president is guilty of high crimes and misdemeanours” – the benchmark for impeachment.

“It’s important that we not have a lawless administration and a lawless president,” the New York Democrat Jerrold Nadler told Fox News Sunday. “And it’s important that people see what we’re doing and what we’re dealing with.”

Nadler’s committee would initiate impeachment proceedings. Mueller, a former director of the FBI, will also appear before the intelligence panel.

The administration must be held accountable and no president can be above the law Jerrold Nadler

“The report presents very substantial evidence that the president is guilty of high crimes and misdemeanours,” Nadler said, “and we have to present, or let Mueller present those facts to the American people and then see where we go from there because the administration must be held accountable and no president can be above the law.”

Mueller was appointed in May 2017, submitted his report in March 2019 and saw it released with redactions a month later.

Attorney general William Barr released a brief summary in which he said conspiracy between Trump and Moscow had not been proven and instances of possible obstruction of justice – the report details 11 by the president or his campaign – were not sufficient to establish that an offence had been committed.

Mueller, who said in his report he did not exonerate Trump, subsequently gave a press conference in which he said his work should speak for itself. Most took that to mean he did not pursue the obstruction charges in part because of a justice department opinion which holds that a sitting president cannot be indicted.

On Sunday, Adam Schiff of California, the House intelligence chair, told CBS’s Face the Nation: “It’s been clear from Bob Mueller that he felt and the justice department feels bound by this Office of Legal Counsel opinion that you can’t indict a sitting president.”

The Russia investigation is not the only subject of discussion of impeachment and whether Trump has committed “high crimes and misdemeanours”, a standard not defined in the constitution and thereby forever the subject of debate.

In the case of now-closed investigations of campaign finance violations involving former Trump lawyer Michael Cohen making payments to women who claim affairs with Trump, Schiff said the president was “essentially an unindicted co-conspirator”.

He is not above the law. He may have a temporary reprieve while he occupies that office Adam Schiff

“He has been identified as ‘individual one’,” he said, “the person who directed Michael Cohen to commit this fraudulent campaign scheme. And I assume this all means that in the Southern District of New York, the case will be reopened when he leaves office.”

Schiff also said that though Cohen is in prison, Trump is “not above the law. He may have a temporary reprieve” while he is in the White House.

Among Democrats, debate rages on about whether impeachment is merited or politically desirable as the 2020 election approaches. Pro-impeachment opinion is strong among supporters and an increasing number of elected officials, contenders for the presidential nomination among them.

Mueller’s testimony was initially set for 17 July. In a tense political standoff, other demands by House committees for testimony from Trump aides and allies have been blocked by the White House.

Trump, who has repeatedly and inaccurately claimed exoneration, said this week he would not watch Mueller’s testimony and accused Democrats of “just playing games”.

On Sunday, he tweeted a nonspecific but familiar complaint about “presidential harassment” and focused on his racist attacks on four progressive Democratic congresswomen.

Republicans who will question Mueller have tried to dampen expectations.

The hearing will be “like an old TV show that you watched years ago”, Doug Collins of Georgia, the top Republican on the judiciary committee, told the Associated Press. “After a few minutes you could quote what the characters could say, and nothing is new any more. Frankly, the American people have moved on.”

Jerrold Nadler arrives to the House floor for a vote. Photograph: REX/Shutterstock

Unsurprisingly, David Ciciline, a Rhode Island Democrat on the same panel, disagreed. He told the AP the hearing would be “the first opportunity for the American people to hear directly from Mr Mueller about what he found about Russian interference in the American presidential election and efforts by the president to impede, undermine or stop the investigation.”

He added: “I do think that the contents of the report are so significant, and so damning, that when Mr Mueller brings them to life, and actually tells the American people … it will have an impact.”

Accordingly, Democrats have been preparing intensely.

Jamie Raskin of Maryland, another member of the judiciary committee, told the AP: “There are still millions of people who think, absurdly, that there is no evidence of presidential obstruction or collusion in the report.”

That, he said, was because Barr and Trump have created a “fog of propaganda”. Raskin said: “We just want to clear the fog.”

Nadler told Fox News Sunday any Republican questioning of Mueller about supposed FBI misconduct, an attack line meant to discredit the special counsel’s work, would be an “irrelevancy” and a waste of time.

“What’s before the American people is the conduct of this president,” he said.

• This article was amended on 22 July 2019 because an earlier version described David Ciciline as a New Jersey Democrat. This has been corrected to say a Rhode Island Democrat.