WASHINGTON (Reuters) - President Donald Trump’s pick for acting attorney general, Matt Whitaker, is a close friend of Trump’s 2016 election campaign co-chair, and a former government ethics chief said the friendship makes Whitaker unable to oversee impartially a politically charged investigation into the campaign.

Matthew Whitaker, named on Wednesday to replace Jeff Sessions, will directly oversee Special Prosecutor Robert Mueller’s investigation into possible links between Trump’s campaign team and Russian officials.

Whitaker publicly criticized Mueller’s investigation before he was hired as Sessions’ chief of staff last year.

Sam Clovis, who was co-chair of Trump’s 2016 campaign and has testified before the grand jury in the Mueller investigation, said he and Whitaker became good friends when they ran against each other as Republicans in a 2014 Senate primary campaign in Iowa. Whitaker also later served as the chairman of a Clovis campaign for state treasurer.

In an interview with Reuters, Clovis said Whitaker is “a wonderful man” and “a dear friend.” He added that Whitaker was a “sounding board” for him when Clovis worked for Trump’s campaign.

Walter Shaub, who was director of the U.S. Office of Government Ethics for four years before resigning in July 2017, said the friendship between Whitaker and Clovis should disqualify Whitaker from supervising the Mueller investigation.

“Whitaker has to recuse himself under DOJ’s regulation requiring recusal if you have a personal or political relationship with someone substantially involved in conduct that is the subject of the investigation or prosecution,” Shaub told Reuters.

Department of Justice spokesperson Sarah Isgur Flores declined to comment.

A DOJ regulation cited by Shaub states that employees “may not supervise prosecutions or investigations that involve someone with a personal or political relationship.”

FILE PHOTO: Chief of Staff to the Attorney General Matthew Whitaker attends a roundtable discussion with foreign liaison officers at the Justice Department in Washington, U.S., August 29, 2018. REUTERS/Allison Shelley/File Photo

It says the possible conflict can be set aside if the employee’s supervisor judges that the relationship does not affect the official’s impartiality or create the public perception of a conflict.

Trevor Potter, a former Republican commissioner on the Federal Election Commission who now leads the Campaign Legal Center, which advocates for more transparency in elections, said the question of whether Whitaker should recuse himself depends on Clovis’ status in the Mueller investigation.

“If he has a close personal friendship with someone who is involved in the investigation because of his role in the Trump campaign, then that would present a recusal issue,” Potter said.

Mueller has not publicly identified any of the targets of his investigation.

Paul Rosenzweig, a fellow at the non-partisan R Street Institute which specializes in public policy, said he believed Whitaker’s friendship with Clovis is “probably not a problem” but that Whitaker should ask the DOJ’s Professional Responsibility and Accountability Office whether he has a conflict.

While he still led the government ethics office, Shaub last year advised the Justice Department to require Sessions’ recusal from the Mueller probe because Sessions had been a senior adviser to Trump during the election campaign.

Sessions’ decision to recuse himself infuriated Trump. After months of publicly criticizing his attorney general, Trump asked him to resign on Wednesday.

The White House did not respond to a request for comment.

Whitaker’s selection by Trump as acting attorney general drew sharp criticism from Democratic lawmakers, who described it as an attempt by the president to undermine and possibly end Mueller’s investigation.

Mueller’s team has netted convictions and guilty pleas from several Trump campaign staff members and advisers.

Trump has repeatedly denied any collusion with Russia, and describes Mueller’s investigation as a “witch hunt.”

Moscow has denied U.S. security agencies’ allegations that it interfered in the election in a bid to help Trump.

A former U.S. attorney and conservative commentator, Whitaker last year wrote an opinion piece for CNN arguing that Mueller would be going too far if he investigated the Trump family’s finances.

Two months after the article was published, Whitaker went to work at the Department of Justice.