Senate Democrats on Friday sent a letter to the Justice Department's inspector general calling for an ethics probe to be opened into Acting Attorney General Matthew Whitaker.

The letter focuses on Whitaker's refusal to recuse himself from special counsel Robert Mueller's investigation into Russian election interference.

The criticism directed at Whitaker raises questions about the challenges attorney general nominee William Barr might face during his upcoming confirmation hearing.

Senate Democrats on Friday sent a letter to the Justice Department's inspector general calling for an ethics probe to be opened into Acting Attorney General Matthew Whitaker.

The letter, signed by every Democrat on the Senate Judiciary Committee, zeroed in on Whitaker's refusal to recuse himself from special counsel Robert Mueller's investigation into Russian election interference and the Trump campaign's alleged collusion, The Daily Beast reported.

There have been broad calls for Whitaker to recuse himself from the probe, but he's made no moves in that direction.

The letter comes not long before Trump's nominee for attorney general, William Barr, heads to the Senate for his confirmation hearing.

Read more: Meet William Barr: What you need to know about the possible once and future attorney general

"Acting Attorney General Matthew Whitaker's decision to disregard the advice of career DOJ ethics officials to recuse himself from oversight of Special Counsel Robert Mueller indicates that DOJ’s internal ethics procedures have failed," the senators said in the letter. "Not only does this raise serious concerns about Mr. Whitaker’s current actions as Acting Attorney General, but it also calls into question DOJ's ethics procedures to assess and address potential conflicts of William P. Barr, President Trump's nominee to be Attorney General."

Acting Attorney General Matthew G. Whitaker, gives brief remarks to state and local law enforcement on efforts to combat violent crime and the opioid crisis at the U.S. Courthouse Annex, on November 14, 2018 in Des Moines, Iowa. Whitaker was appointed as acting attorney general after previous attorney general Jeff Sessions was forced out out of the job. Steve Pope/Getty Images

Whitaker's elevation from former Attorney General Jeff Sessions' chief of staff to acting attorney general was controversial from the start, particularly in relation to his past criticism of the Mueller investigation.

Read more: In a 'self-defeating and self-incriminating' slipup, Trump just indicated he installed Matthew Whitaker to kill the Russia probe

Sessions resigned in early November at the request of the president after repeatedly facing criticism from Trump over recusing himself from the Russia probe.

The Justice Department did not immediately respond to a request for comment from INSIDER.