WASHINGTON — In the summer of 2017, when Matthew Whitaker was a conservative legal commentator on CNN, he repeatedly expressed reservations about special counsel Robert Mueller's investigation, at one point musing about how a clever attorney general could secretly starve it of funds.

Two years before, Whitaker, a former Iowa federal prosecutor, was the campaign chairman for an Iowa politician who later became an important witness in Mueller's probe of alleged collusion between the Trump campaign and Russia.

Now, Whitaker is the acting attorney general, and one of his jobs is to supervise the Mueller investigation. He has shown no indication that he will heed the advice of Democrats and legal ethicists, who say there is a clear case that he should recuse himself from that role. There is no legal mechanism to force his recusal.

"Based on what Mr. Whitaker has said in the past about the Mueller investigation, his assumption of responsibility over the investigation certainly raises the appearance of impropriety," said Mary McCord, who headed the Justice Department's National Security Division under President Obama from 2016 to 2017. "I would hope that, at a minimum, he would consult with ethics experts at the Department before assuming that responsibility."

Whitaker, a physically imposing man who played tight end for the University of Iowa team that lost the 1991 Rose Bowl, served most recently as chief of staff for Jeff Sessions, who was forced out by President Donald Trump on Wednesday.

In the Justice Department, Whitaker was viewed as an agent of the White House, one administration official and one former U.S. attorney told NBC News.

"It's no secret where his loyalties lie — they're with the White House more than with the Department," the official said.

Instead of turning to Deputy Attorney General Rod Rosenstein to become the acting attorney general, Trump placed Whitaker in that role. Rosenstein, a career public servant, had been supervising the Mueller investigation because Sessions had been recused, due to his prominent role in the Trump presidential campaign. With Sessions gone, the acting attorney general retains the role of overseeing Mueller — approving budgets, and signing off on new investigative steps or indictments. It is believed the acting attorney general would have to approve the politically fraught move of issuing a subpoena for the president's testimony — unless Mueller has already secured one with Rosenstein's OK.

As a commentator, Whitaker expressed opinions on exactly the issues about which he may now have to decide.

For example, in an Aug. 2017 CNN op-ed, Whitaker wrote that Mueller's investigation was at risk of becoming a "witch hunt." He added, channeling comments, by Trump, that it would be "dangerously close to crossing [a] red line" if Mueller looked into Trump's finances.

Before that, in July, Whitaker brainstormed in a television appearance about how Trump could constrain Mueller.

"I could see a scenario where Jeff Sessions is replaced with a recess appointment," Whitaker said, "and that attorney general doesn't fire Bob Mueller, but he just reduces his budget to so low that his investigation grinds to almost a halt."

That same month, in another appearance, Whitaker defended Donald Trump Jr.'s decision to take a meeting with a Russian lawyer at Trump Tower in June 2016, after he was promised incriminating information about Hillary Clinton from the Russian government.

"You would always take the meeting," he said.

In an Aug. 17 tweet, Whitaker seemed to criticize the FBI's search of the home of former Trump campaign chairman Paul Manafort, who has since pleaded guilty and is cooperating with Mueller.

"Do we want our Gov't to 'intimidate' us? Hmm," he tweeted, linking to a Fox News story with the headline "FBI's Manafort raid included a dozen agents, 'designed to intimidate,' source says."

And in May 2017, speaking on Fox News host Sean Hannity’s radio show, Whitaker opined that the president did not obstruct justice when he urged then-FBI director James Comey to stop investigating his then-national security adviser, Mike Flynn.

“That doesn’t rise to the level of obstruction of justice, and it doesn’t sound to me based on what’s been reported that Jim Comey, as he sat there, believed that the president was telling him to stop the investigation,” Whitaker said. “if all he did was make a mere suggestion and not an outright command, I don’t think that rises to the level of obstruction.”

Do we want our Gov't to "intimidate" us? Hmm -FBI's Manafort raid incl. a dozen agents, 'designed to intimidate,' | https://t.co/KAHQq26q2M — Matt Whitaker 🇺🇸 (@MattWhitaker46) August 25, 2017

Legal experts say there are two grounds on which Whitaker should remove himself from any involvement in the Mueller investigation: His relationship with Sam Clovis and his past statements criticizing the investigation.

A Justice Department regulation says that no employee should participate in an investigation "if he has a personal or political relationship with any person or organization substantially involved in the conduct that is the subject of the investigation."

Clovis testified before the Mueller grand jury, and he was listed as an unnamed "campaign supervisor" in the indictment of former Trump campaign aide George Papadopoulos, who pleaded guilty to lying to the FBI.

Clovis was present at a meeting, along with Sessions and Trump, during which Papadopoulos says he broached the idea of Trump meeting with Russian President Vladimir Putin.

Both Clovis and Whitaker ran in a Republican primary for the U.S. Senate in Iowa in 2014, and during that time they became "great friends," Clovis said in a later statement.

"I respect and admire Dr. Clovis," Whitaker was quoted as saying. "His life is an example of strong service to God and service to country.

Kathleen Clark, a law professor and ethics expert at Washington University, says that friendship would appear to be grounds for recusal. But equally problematic, she says, are Whitaker's statements suggesting he is not impartial about the Mueller investigation.

Federal ethics regulations that apply government-wide state that a government employee should consider stepping aside in a matter when "the circumstances would cause a reasonable person with knowledge of the relevant facts to question his impartiality in the matter."

"I believe that there are circumstances that would raise a question in the minds of a reasonable person about Whitaker's impartiality in the Russia investigation," Clark said. "We as the public have a right to be assured that the people who are participating in that investigation are impartial."

But there is no enforcement mechanism. A former Justice Department official said it is up to Whitaker to seek the advice of ethics officials in the Office of Government Ethics or the Justice Department's ethics office.

"It's incumbent upon the person," the official said. "There is no ethics policeman wandering around the Department."