WASHINGTON — President Trump went out of his way on Friday to distance himself from Matthew G. Whitaker, his choice to replace Jeff Sessions as attorney general, saying repeatedly that he did not know Mr. Whitaker and had not spoken to him and emphasizing that the new attorney general was merely “there in an acting position.”
“I don’t know Matt Whitaker,” Mr. Trump told reporters as he left Washington for a weekend trip to Paris. But the president stressed that he did know Mr. Whitaker’s reputation well, calling him “a very respected man.”
The faint presidential praise came just two days after Mr. Trump unceremoniously dumped Mr. Sessions, putting Mr. Whitaker in charge of the Justice Department and the special counsel’s investigation into Russian interference in the 2016 election. Since then, criticism has mounted about Mr. Whitaker’s lack of credentials to be the nation’s top law enforcement official, along with allegations that he has conflicts of interest that should keep him from overseeing the Russia inquiry.
Senator Chuck Schumer of New York, the chamber’s top Democrat, sent a letter to Mr. Trump on Friday saying that the appointment of Mr. Whitaker, who was chief of staff to Mr. Sessions, was unconstitutional.
“I am not aware of any precedent for appointment of an official who has not been confirmed by the Senate to serve as acting attorney general,” Mr. Schumer wrote.
But in a phone call in recent days, Mr. Whitaker told Senator Charles E. Grassley of Iowa, the Republican chairman of the Judiciary Committee, that a 2003 Office of Legal Counsel opinion held that the president could make such an appointment.
“He said they had the Office of Legal Counsel look into it, and they had precedent,” Mr. Grassley said in an interview.
In addition, the president’s claim that he did not know Mr. Whitaker was called into question by Mr. Trump’s own words from just about a month ago, when he said in a “Fox & Friends” interview: “I can tell you Matt Whitaker’s a great guy. I mean, I know Matt Whitaker.”
Mr. Whitaker has also visited the Oval Office several times and is said to have an easy chemistry with the president, according to people familiar with the relationship. And the president has regarded Mr. Whitaker as his eyes and ears at the Justice Department.
The United States Court of Appeals for the District of Columbia Circuit also wants to know what impact Mr. Whitaker will have on the investigation. On Friday, the court requested a report from both the Justice Department and the Mueller team “addressing what, if any, effect” Mr. Whitaker’s appointment has on the case, which was previously being overseen by Rod J. Rosenstein, the deputy attorney general.
As Mr. Sessions’s chief of staff, Mr. Whitaker helped stabilize the tense relationship between Mr. Trump and Mr. Sessions, according to officials who were familiar with the dynamic.
Mr. Whitaker attended meetings at the White House; in some cases, the president and the attorney general were both there, officials who were also present said.
In August, when the relationship between Mr. Trump and Mr. Sessions hit a low point and the two men exchanged a rare public tit-for-tat, Mr. Whitaker attended a meeting on prison overhaul with both of them later that day.
Yet, on Friday morning, Mr. Trump said four times that he did not know Mr. Whitaker.
Democrats have called for Mr. Whitaker to recuse himself from the special counsel investigation over what they see as his conflicts of interest, including his public criticism of the inquiry and his ties to a witness in the investigation. The witness, Sam Clovis, is a former Trump campaign aide and a former candidate for Iowa state treasurer. In 2014, Mr. Whitaker was the chairman of Mr. Clovis’s unsuccessful campaign.
Mr. Trump suggested on Friday that he was a little surprised that Mr. Whitaker’s appointment would draw such criticism.
“You didn’t have any problems with Matt Whitaker when he was working for Jeff Sessions,” Mr. Trump said.
“No matter who I put in, they go after him,” Mr. Trump said of his appointees. “It’s very sad.”
Mr. Grassley, who recommended Mr. Whitaker to serve as United States attorney during the George W. Bush administration, said he sympathized and saw no reason he should recuse himself from overseeing the Russia case.
“Let’s look at why Sessions recused himself — he talked to the Russian ambassador a couple times,” Mr. Grassley said. “Whitaker has not done that. So what would be the basis?”
One criticism of Mr. Trump’s decision to appoint Mr. Whitaker is that it goes against the Justice Department’s succession plan to turn first to the deputy attorney general — Mr. Rosenstein — when the top job is vacant. The president has publicly criticized Mr. Rosenstein, as well.
Senator Richard Blumenthal, Democrat of Connecticut, said in an interview on Friday that he was considering filing a lawsuit to contest Mr. Whitaker’s appointment, most likely on grounds raised by Mr. Schumer.
“There are a number of very serious constitutional issues raised by this deeply flawed appointment,” Mr. Blumenthal said.
Mr. Trump, however, has authorities provided in a separate law that gives him options for making temporary appointments without Senate confirmation.
Speaking to reporters on Friday morning, Mr. Trump bristled at criticism that Mr. Whitaker was not the right choice to run the department, because, as Mr. Schumer said, Mr. Whitaker was not currently serving in a Senate-confirmed position.
“Mueller was not Senate confirmed. So, he’s doing a report. He wasn’t Senate confirmed,” Mr. Trump said. “Whitaker was Senate confirmed. And now he doesn’t need this, but he was Senate confirmed at the highest level when he was the U.S. attorney from Iowa.”
Mr. Whitaker was confirmed by the Senate when he served as the United States attorney for the Southern District of Iowa during the George W. Bush administration. But so was Mr. Mueller. In 2001, Mr. Mueller was confirmed by the Senate to be the F.B.I. director. And he went through previous Senate confirmations to serve as the United States attorney in the Northern District of California as well as when he served as the assistant attorney general at the Justice Department in the early 1990s.
The special counsel position is not one that requires Senate confirmation.