As President Trump has sought to overturn the election results, his personal lawyers paraded themselves before television hosts, state elections officials and anyone else willing to entertain their baseless claims of voter fraud.
But behind the scenes, a longtime conservative lawyer named Cleta Mitchell quietly helped. Her work for Mr. Trump drew widespread attention for the first time over the weekend, when a recording was released of an hourlong call in which Mr. Trump threatened Georgia elections officials with “a criminal offense” if they failed to “find” enough votes to change the state’s presidential results.
On the call, Ms. Mitchell repeatedly jumped in to help Mr. Trump, showing an intimate level of involvement in his efforts as they made baseless claims about the election and pressed Georgia officials to hand over election data.
Ms. Mitchell is a partner at the law firm Foley & Lardner, which has over 1,000 lawyers and an office in nearly every major city in the United States and represents large corporations such as CVS Pharmacy. Her presence on the call stood out because Mr. Trump has struggled to attract high-profile lawyers to aid his attempts to overturn the election, instead relying on a group that has included Rudolph W. Giuliani, the former New York City mayor who has pushed outlandish claims in defense of Mr. Trump throughout his presidency, and Sidney Powell, who has espoused conspiracy theories.
Ms. Mitchell has been advising the president for weeks, according to a person familiar with the matter.
In the day after the audio emerged, Foley & Lardner sought to distance itself from Ms. Mitchell, saying in a statement on Monday that its lawyers were expected to refrain from representing or advising anyone in the election. The firm said it was examining Ms. Mitchell’s role on Mr. Trump’s legal team.
“We are aware of, and are concerned by, Ms. Mitchell’s participation in the Jan. 2 conference call and are working to understand her involvement more thoroughly,” the firm said.
Ms. Mitchell did not return an email seeking comment.
Although she is affiliated with an established law firm, Ms. Mitchell fits the mold of many lawyers who have advised Mr. Trump during his presidency as he faced a special counsel investigation and impeachment and has now taken on a specious effort to overturn the vote.
Ms. Mitchell has for years represented a range of conservatives such as Scott Pruitt, Mr. Trump’s former head of the Environmental Protection Agency, and organizations like the National Rifle Association. In recent years, she represented conservative Tea Party groups that accused the I.R.S. of wrongly targeting them.
During the Russia investigation, Ms. Mitchell publicly attacked the special counsel, Robert S. Mueller III, in the news media, questioning why his team was not examining Hillary Clinton instead.
Ms. Mitchell, 70, started her political career as a Democrat, winning election to the Oklahoma House of Representatives in her mid-20s and serving for nearly a decade before unsuccessfully campaigning for lieutenant governor in 1986.
She came to Washington as an activist pushing for term limits and registered as a Republican in 1996. She established herself as one of the one of the party’s leading election and nonprofit lawyers, representing its congressional campaign arms and several of its candidates, as well as groups that back Republicans, including the N.R.A., where she also served on the board.
But unlike most traditional political lawyers, Ms. Mitchell maintained a public profile supporting candidates and causes, earning a reputation as a firebrand. She was a leading critic of the I.R.S.’s treatment of nonprofit groups associated with the Tea Party movement during the Obama administration and of state and local coronavirus restrictions that religious groups opposed last year.
During the Trump administration, Ms. Mitchell has also represented the nonprofit of the president’s former chief strategist Stephen K. Bannon, which has been scrutinized by federal prosecutors in Manhattan as part of a broad investigation into whether Mr. Bannon defrauded donors.
At one point in the call over the weekend, Mr. Trump brought up a baseless claim about ballots from Atlanta that were for President-elect Joseph R. Biden Jr.
“Does anybody know about it?” Mr. Trump asked.
“I know about it, but —” Ms. Mitchell said before she was interrupted by the president.
“OK, Cleta, I’m not asking you. Cleta, honestly. I’m asking Brad,” Mr. Trump said, referring to Secretary of State Brad Raffensperger of Georgia.
Maggie Haberman contributed reporting. Sheelagh McNeill contributed research.