WASHINGTON (Reuters) - U.S. Senate Democrats are considering legal action over President Donald Trump’s appointment of a new acting attorney general, congressional sources said on Friday, as some outside experts called the move unconstitutional.

Trump on Wednesday named Matthew Whitaker to replace former Attorney General Jeff Sessions, who was forced out after months of attacks by Trump for recusing himself from an ongoing probe into Russian meddling in the 2016 presidential election.

The move made Whitaker supervisor of the investigation, which has hung over Trump’s presidency. Whitaker has criticized the probe in the past as too wide-ranging, which has raised concerns among Democrats that Sessions’ ouster and Whitaker’s appointment might be precursors to Trump moving to end it.

Senate Democrats were considering suing Trump, the sources said, on the grounds that, in naming Whitaker, the president ignored a statutory line of succession at the Justice Department and deprived senators of their constitutional “advice and consent” role on some presidential appointments.

“The only two paths to that office are regular succession, and advice (and) consent,” said a source close to the Senate Judiciary Committee.

Democratic Senator Richard Blumenthal told Reuters late on Friday he was “considering action that might be brought against an interim appointment that violates the normal statutory line of succession and raises very serious constitutional questions.”

He said he was speaking only for himself and he hoped Republicans might join as plaintiffs if a lawsuit goes forward.

The Appointments Clause of the U.S. Constitution states that some senior government officials, known as “principal officers,” must be confirmed by the Senate.

A spokesman for Senate Judiciary chairman Chuck Grassley said Trump had the authority to appoint Whitaker as acting attorney general temporarily, even though he had not been confirmed by the Senate.

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Such appointments can be done for senior officials who have worked in the department for at least 90 days and can last for up to 210 days, spokesman George Hartmann said.

As the minority party in the Senate, Democrats might need some Republican support to have legal standing to sue Trump under the Appointments Clause, said Andrew Wright, who was a White House lawyer under former President Barack Obama.

The source close to the Senate Judiciary Committee said Democrats were unsure whether they would reach out to Republicans to join the lawsuit, but added it was “not likely.”

Republican Senator Lindsey Graham, who earlier this year introduced legislation to protect Special Counsel Robert Mueller, who is conducting the probe, said Whitaker did not pose a threat to his work.

“Mueller will be allowed to do his job,” Graham said in a Friday interview on Fox News Radio.

John Yoo, a former Justice Department lawyer in the George W. Bush administration, said “the Supreme Court made clear that the Attorney General is a principal officer” in a 1998 case.

“Therefore, Whitaker cannot serve as acting Attorney General ... Any other officer in the Justice Department who was appointed through advice and consent can serve, including the Deputy AG, the solicitor general, and the assistant AGs,” said Yoo, now a law professor at the University of California, Berkeley.

Senate Democratic leader Chuck Schumer said Deputy Attorney General Rod Rosenstein, a career Justice Department official already confirmed by the Senate, should have been named the new attorney general.

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RESTRICTIONS ON TRUMP

U.S. intelligence agencies have said Russia interfered in the 2016 election in an attempt to tip it towards Trump and away from his Democratic opponent, former Secretary of State Hillary Clinton.

Republican Senator Susan Collins said Mueller must be allowed to complete his investigation into whether Trump’s campaign colluded with Moscow.

Trump has repeatedly said there was no collusion, and has slammed the probe as a “witch hunt.” Russia has denied interfering.

“I am concerned about comments that Acting Attorney General Matthew Whitaker has made regarding the Special Counsel and the parameters of his investigation,” Collins said in a statement.

“We should bring to the Senate floor legislation that would put restrictions on the ability of President Donald Trump to fire the Special Counsel.”

Speaking to reporters at the White House before he left on a trip to Paris, Trump defended his choice of Whitaker, saying the former U.S. attorney for the southern district of Iowa had an excellent reputation and came highly recommended by former Iowa Governor Terry Branstad, who is now Trump’s ambassador to China.

In a late night Tweet on Friday, Trump reiterated that he did not know Whitaker, but that he was very highly thought of by Iowa senators Joni Ernst and Chuck Grassley and Branstad.

“I feel certain he will make an outstanding Acting Attorney General!,” Trump said.

Trump said on Friday he had not discussed the Mueller probe with Whitaker before appointing him.

A spokeswoman for the Justice Department has said Whitaker would oversee all matters under its jurisdiction, including the Mueller investigation. Democrats have called on Whitaker to recuse himself.