Sen. Chuck Grassley Charles (Chuck) Ernest GrassleyGOP lawmakers distance themselves from Trump comments on transfer of power The Hill's 12:30 Report: Ginsburg lies in repose Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy MORE (R-Iowa) is demanding to know why Andrew McCabe hasn't recused himself from a probe into former Trump adviser Michael Flynn, arguing the acting FBI director is creating the "appearance of political bias."

Grassley sent Deputy Attorney General Rod Rosenstein a letter this week asking what steps he will take to "address the appearance of political and other conflicts of interest."

"[The] evidence and the failure to recuse calls into question whether Mr. McCabe handled the Flynn investigation fairly and objectively, or whether he had any retaliatory motive against Flynn for being an adverse witness to him in a pending proceeding," Grassley wrote.

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The letter marks the latest escalation of Grassley's long-running feud with McCabe, whom he accused in early May of casting a "significant cloud of doubt" over the FBI. He called for McCabe to not be allowed to take over the bureau in the wake of former FBI Director James Comey's firing.

According to a Circa news report, cited by Grassley, three former FBI officials heard McCabe making "disparaging remarks about Flynn before and during the time ... [he] emerged as a figure in the Russia case.”

Flynn also offered to testify on behalf of Robyn Gritz, a former FBI official who was fired, in a lawsuit accusing the FBI of sexual discrimination, according to a separate report from Circa, a website that is closely followed by conservative media.

Grassley also pointed to McCabe's refusal to step back from the FBI's investigation into Virginia Gov. Terry McAuliffe (D), a longtime Clinton ally.

The senator is giving the bureau roughly two weeks to respond to this week's letter, adding that it didn't respond to his May questions.

The FBI turned over documents in late 2016 outlining when McCabe would need to recuse himself but, according to Grasley, they included redactions.

"Oddly, Mr. McCabe was the approval authority for his own recusal memo. ... The Committee requires unredacted copies of the document for its inquiry," he said.

Trump previously cited donations to a Virginia state Senate race involving McCabe's wife to suggest the FBI's investigation into former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonBloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE's email server was politically biased.

The FBI told the fact-checking website PolitiFact that at the time of the contribution, McCabe wasn't directly involved with the FBI's investigation.