As Donald Trump tried again to distance himself from Matthew Whitaker, his own choice as acting attorney general after he fired Jeff Sessions, the House minority leader, Nancy Pelosi, said the appointment “does violence to the constitution and the vision of our founders”.

Critics have said Whitaker’s appointment is unconstitutional, as he has not been confirmed by the Senate. His legal qualifications have been questioned and his background examined by media outlets including the Guardian, which reported that he had a paid advisory role with a company that scammed military veterans out of their retirement savings.

It was reported on Friday that the FBI is investigating that company.

Whitaker now oversees the Mueller investigation into Russian election interference, potential collusion between Trump aides and Moscow, and potential obstruction of justice by the president. He has criticised the investigation in the media.

Trump’s attempts to deny knowing Whitaker stand against extensive reporting that he does.

I mean, I know Matt Whitaker Donald Trump to Fox News, October

The Associated Press reported that two Republicans close to the president said he had enjoyed Whitaker’s TV appearances and the two had struck a bond. Those TV appearances included one on CNN in which Whitaker suggested the investigation led by special counsel Robert Mueller could be starved of resources.

Speaking to Fox News last month, meanwhile, Trump called Whitaker “a great guy” and said: “I mean, I know Matt Whitaker.” In September, the New York Times reported that Whitaker had been a frequent visitor to the Oval Office and had developed an “easy chemistry” with the president.

Nonetheless, on Friday the president repeatedly claimed not to know him.

Leaving the White House, Trump told reporters: “I don’t know Matt Whitaker.” Later, from France, he tweeted again that he “did not know” him and claimed “no social contact” with him.

Trump tweeted: “Matthew G Whitaker is a highly respected former US attorney from Iowa. He was chosen by Jeff Sessions to be his chief of staff. I did not know Mr Whitaker. Likewise, as chief, I did not know Mr Whitaker except primarily as he traveled with AG Sessions. No social contact …”

Trump then added a list of referees for Whitaker, whom he said was “very highly thought of” by Iowa senators Joni Ernst and Chuck Grassley, former Iowa governor turned China ambassador Terry Branstadt, “Leonard Leo of Federalist Society” – the head of the rightwing group which helped Trump pick his two supreme court justices – “and many more”.

“I feel certain he will make an outstanding acting attorney general,” Trump said.

Pelosi spoke in an interview to be broadcast on CBS Face the Nation on Sunday. Whitaker’s appointment, said the Democrat who is line to be speaker of the House in the next Congress, “does violence to the constitution and the vision of our founders to appoint such a person in such a manner to be the chief legal officer in our country. And that’s bipartisan.”

Mitch McConnell, the Senate majority leader, said on Friday he thought Whitaker’s time as acting attorney general would be brief. McConnell also said he thought Mueller would be left to complete his work.

Whitaker is a Republican loyalist. Before joining the justice department last year, he wrote a column for CNN in which he said Mueller would be outside his mandate if he investigated Trump family finances. He also gave a radio interview in which he maintained there was no evidence of collusion between Trump aides and Moscow.

Matthew Whitaker, seen in August. Photograph: Allison Shelley/Reuters

He also tweeted an ex-prosecutor’s opinion piece that described a “Mueller lynch mob”, which he said was “worth a read”.

According to the Republicans who spoke to the Associated Press on condition of anonymity, Trump told associates he felt Whitaker would be loyal and would not have recused himself from the Russia investigation as Sessions did over campaign meetings with Russian officials.

On Friday, Trump said he had not spoken to Whitaker about Mueller’s investigation, which has been overseen by the deputy attorney general, Rod Rosenstein. Rosenstein said Whitaker was “a superb choice for attorney general”.

Of the scrutiny Whitaker is facing, Trump said: “It’s a shame that no matter who I put in they go after. He was very, very highly thought of, and still is highly thought of, but this only comes up because anybody that works for me, they do a number on them.”

There have also been reports about Whitaker’s past comments questioning the power of the federal judiciary, and about his stated belief that all judges should be Christian.

Whitaker has not spoken publicly. He sent a justice department-wide note after his appointment, in which he said: “As we move forward, I am committed to leading a fair department with the highest ethical standards, that upholds the rule of law, and seeks justice for all Americans.”

Trump has not said whom he will nominate to replace Sessions. The former New Jersey governor Chris Christie is said to be a candidate, as are the labor secretary, Alex Acosta, and health secretary, Alex Azar, among others.