A lawyer advising President Trump in recent weeks has resigned from her law firm after it was revealed that she participated in the call where Mr. Trump pressured Georgia officials to help him reverse the state’s election results, the firm said in a statement on Tuesday.

The lawyer, Cleta Mitchell, has been advising Mr. Trump despite a policy of her firm, Foley & Lardner, that none of its lawyers should be representing clients involved in relitigating the presidential election.

“Cleta Mitchell has informed firm management of her decision to resign from Foley & Lardner effective immediately,” the firm said in a statement. “Ms. Mitchell concluded that her departure was in the firm’s best interests, as well as in her own personal best interests. We thank her for her contributions to the firm and wish her well.”

Foley & Lardner had begun to distance itself from Ms. Mitchell shortly after the call was first reported by The Washington Post on Sunday. As Mr. Trump has made increasingly specious claims about the election, he has been unable to attract high-profile, establishment lawyers to back his cause.

Ms. Mitchell was among several Trump aides who joined the call on Saturday in which the president threatened Georgia’s secretary of state, Brad Raffensperger, with “a criminal offense” if he failed to “find” enough votes to change the state’s presidential results.

Ms. Mitchell has been involved in representing far-right groups and conservatives for many years. She served on the board of the National Rifle Association and represented Tea Party groups that claimed they were illegally targeted by the I.R.S.