Republicans plan to lodge a complaint against one of Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE's attorneys in connection with the investigation surrounding the Democratic presidential nominee's use of a private email server while serving as secretary of State.

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Officials with the Republican National Committee (RNC) plan to lodge the complaint Monday with the District of Columbia Bar against Cheryl Mills, a Clinton confidante.

They are alleging that Mills — who served as chief of staff when Clinton led the State Department — violated the bar's rules of conduct during the FBI's investigation.

“The FBI’s grant of immunity to Ms. Mills highlights the apparent conflict of interest in her representation of Secretary Clinton,” Raj Shah, the RNC's research director, wrote in a letter.

The letter said Mills might have broken a rule that prevents lawyers from accepting "other employment in connection with a matter which is the same as, or substantially related to, a matter in which the lawyer participated personally and substantially as a public officer or employee."

FBI director James Comey has defended the investigation.