Sen. Chuck Grassley Charles (Chuck) Ernest GrassleyTrump walks back aluminum tariffs on Canada Trump order on drug prices faces long road to finish line GOP chairman to release interim report on Biden probe 'in about a week' MORE (R-Iowa) says that perceptions of potential political interference in FBI investigations are casting a "significant cloud of doubt" over the bureau.

Grassley, who chairs the Senate Judiciary Committee, sent a letter to Rod Rosenstein, the recently confirmed deputy attorney general, raising concerns that FBI Deputy Director Andrew McCabe's involvement in investigations creates an "appearance of political bias."

"Public reports of his meeting with a longtime Clinton and Democrat party fundraiser, Governor Terry McAuliffe [Va.], and his wife’s subsequent campaign for public office being substantially funded by McAuliffe’s organization raise serious questions about his ability to appear impartial," Grassley said in the letter sent to Rosenstein on Tuesday.

The GOP senator added that Rosenstein must "begin to closely supervise and oversee the FBI’s handling of politically charged, high-profile and controversial investigations."

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Grassley has previously focused on McCabe as part of his probe into a controversial opposition research dossier compiled during the campaign against President Trump.

He said in Tuesday's letter that McCabe is also under investigation by the Department of Justice's inspector general "for failing to recuse himself from the Clinton investigation due to his meeting with McAuliffe." McAuliffe is a longtime ally of former President Bill Clinton William (Bill) Jefferson ClintonTrump pulls into must-win Arizona trailing in polls Does Kamala Harris's music matter? President Nancy Pelosi? Don't underestimate what she might do in office MORE and his wife, former Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonBiden courts veterans amid fallout from Trump military controversies Biden looks to shore up Latino support in Florida MLB owner: It's 'very necessary' to vote for Trump MORE.

Grassley pointed to a Wall Street Journal editorial that said most of McAuliffe's donations to a Virginia state Senate race involving McCabe's wife came after the FBI started its probe into Hillary Clinton's use of a private email server while secretary of State.

"The financial and political links between Mr. McCabe and Gov. McAuliffe raise concerns about the appearance of impartiality in the course of not only the Clinton investigation, but the reported McAuliffe investigation, and the ongoing investigation of alleged ties between associates of Mr. Trump and Russia," Grassley added in his letter.

He said that the FBI's rules "demand that he and the FBI take steps to ensure that no appearance of a loss of impartiality undermines public confidence in the work of the Bureau."

Grassley argued that a "significant cloud of doubt" has been cast over the bureau and signaled he wants Rosenstien to detail what steps he will take to ensure the FBI's investigation into Russian interference in the 2016 presidential election isn't "tainted with the appearance of political bias."

He also wants to know what steps Rosenstein will take to ensure apparent leaks about the FBI's investigation are probed "given that several senior FBI officials ... are potential suspects" or how he will guarantee that McAuliffe was or is being fully investigated.

Trump previously used the donation between McAuliffe and McCabe's wife to suggest the FBI's investigation into Clinton's email server was politically biased.

But PolitiFact rated Trump's remark as "mostly false." The FBI told the fact-checking website that at the time of the contribution McCabe wasn't directly involved with the FBI's investigation.

The FBI wrote in a letter to Grassley last month that "the FBI has assessed that there is no basis in law or in fact for such a recusal" from the Russia investigation.

But Grassley signaled he still has concerns about McCabe's role in the FBI's investigation of Russia's meddling in the 2016 election and a potential investigation in "politically motivated leaks" tied to the probe.

He added that the FBI didn't provide "reasoning, rationale, or documentation to support this conclusory statement" when it said McCabe didn't need to recuse himself from the Russia probe.