Cleta Mitchell, who emerged as a key player on the phone call in which Mr Trump asked Brad Raffensperger to “find” thousands of votes for him, resigned from Foley & Lardner on Tuesday.
"Cleta Mitchell has informed firm management of her decision to resign from Foley & Lardner effective immediately,” spokesman Dan Farrell told The Independent.
“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."
The lawyer was identified during the taped conversation released by The Washington Post on Sunday, which caused immediate backlash and calls for a second impeachment of the president.
At the outset of the call, White House Chief of Staff Mark Meadows apparently said Ms Mitchell was “not the attorney of record but has been involved” in the campaign's lawsuits to overturn the election result.
When Mr Trump was told by Mr Raffensperger that the data he had was inaccurate, the president purportedly asked Ms Mitchell, “Well, Cleta, how do you respond to that? Maybe you tell me?”
Ms Mitchell is said to have contested Mr Raffensperger by saying that lawyers “have asked from your office for records that only you have” but had not received them.
The 70-year-old reportedly told friends and clients in an email on Tuesday that she had resigned, blaming "a massive pressure campaign in the last several days mounted by leftist groups" for her resignation.
"With the ever more brazen attacks on conservatives and, most especially, anyone who supports and wants to help President Trump, I realise that a large national law firm is no longer the right platform for me or my law practice," she wrote, according to CNN.
Ms Mitchell confirmed the email to the broadcaster but declined to comment further.
Ms Mitchell has been long acted as a prominent right-wing voice on alleged election fraud, however, according to CNN several senior officials were unaware she was working with Mr Trump until the phone call surfaced.
Foley & Lardner, which has over 1,000 lawyers, had previously said on Monday that it was “concerned” by Ms Michell’s participation in the call and was “working to understand her involvement more thoroughly.”
They stipulated that the firm had made a “policy decision not to take on any representation of any party in connection with matters related to the presidential election results.”
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