U.S. President Donald Trump said Friday he has chosen former attorney general William Barr to once again lead the Justice Department, a role that would put him in charge of the probe into Russian election interference and possible collusion between Moscow and the Trump campaign.

If confirmed by the Senate, Barr would take over from Matthew Whitaker, who has been serving in an acting capacity since Trump forced out Jeff Sessions a month ago. Whitaker had been Sessions's chief of staff.

Barr was "my first choice from Day 1," and "a terrific man, a terrific person, a brilliant man," Trump said, speaking to reporters outside the White House.

Barr, a lawyer who was previously attorney general from 1991 to 1993 under the late president George H.W. Bush, has defended Trump's controversial decision to fire then FBI director James Comey in May 2017 when Comey was leading the Russia probe.

After Comey's firing, special counsel Robert Mueller took over that investigation, which includes any possible collusion between Moscow and Trump's 2016 election campaign, and any potential obstruction of justice. The Russia probe has long infuriated Trump, who calls it a witch hunt and who has denied any collusion or any obstruction of justice.

Barr has said there is more reason to investigate potential wrongdoing by Trump's campaign opponent, Democrat Hillary Clinton, than there is to probe any potential collusion.

....and one of the most highly respected lawyers and legal minds in the Country, he will be a great addition to our team. I look forward to having him join our very successful Administration! —@realDonaldTrump

Mueller, a Republican, was appointed by Deputy Attorney General Rod Rosenstein. Barr has said political donations show Mueller's team of professional prosecutors tilt uncomfortably to the left. On Twitter, Trump calls them the "17 Angry Dems."

"I would have liked to see him have more balance on this group," Barr told the Washington Post in July 2017.

Takes over responsibility for Russia probe

As attorney general, Barr would have ultimate responsibility for the Russia probe, unless he recuses himself. Sessions recused himself from overseeing the investigation.

U.S. intelligence agencies have concluded Moscow worked to influence the election and tip it in Trump's favour. Russia has denied any interference.

Barr's comments on Mueller and Clinton could stir opposition from Senate Democrats, but the nomination will almost certainly not come up for a vote until next year. Republicans will control the chamber with a 53-47 majority in the new Congress convening in January.

"I do think he's worthy of consideration. I am concerned he has said some negative things about the special counsel's office and some of the prosecutors he had in place," Democratic Senator Amy Klobuchar told MSNBC on Thursday after Barr's name surfaced.

Klobuchar is a member of the Senate judiciary committee, which will hold hearings on the nomination.

Supported Comey firing, Clinton investigation

In an opinion piece in the Washington Post last year, Barr argued that Comey usurped the authority of top Justice Department officials when he announced the outcome of an FBI probe into Clinton's use of a private email server when she was secretary of state, and that Trump was right in firing him.

At the time of Comey's announcement, both Clinton and Trump were candidates for president. When Trump fired Comey, the initial reason given by the White House was his poor handling of the FBI investigation into Clinton's emails.

Barr, left, is seen here in November 1991 after being sworn in as Bush's attorney general. (Scott Applewhite/Associated Press)

Barr, however, did not attack the Russia probe itself, which he said was being conducted with a thoroughness that appeared lacking in the Clinton email investigation.

Barr has expressed sympathy for Trump's calls to take a second look at whether Clinton may have broken the law, telling the Washington newspaper in November 2017: "I do think that there are things that should be investigated that haven't been investigated."

That same month, he told the New York Times he thought there was more reason to investigate Clinton for any role she may have played in approving a uranium deal when she was secretary of state than any potential collusion between the Trump campaign and Moscow.

Nauert for UN ambassador

Trump on Friday also said he would nominate U.S. State Department spokesperson Heather Nauert as the next U.S. ambassador to the United Nations, choosing a former TV anchor with little policy experience to lead American diplomacy at the international organization.

Nauert, whose nomination requires U.S. Senate confirmation, is a former Fox News Channel correspondent who had not worked in government before starting at the State Department 18 months ago.

Trump also revealed Friday he would nominate U.S. State Department spokesperson Heather Nauert as the latest U.S. ambassador at the United Nations. (Alex Brandon/Associated Press)

If confirmed, she would replace Nikki Haley, the former South Carolina governor who announced in October that she would step down as ambassador at the end of this year.

Trump also told reporters assembled outside the White House that he would make another staffing announcement on Saturday involving the Joint Chiefs of Staff, but gave no other details.