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SEC Chair Gary Gensler’s pick to serve as the agency’s enforcement director resigned unexpectedly on Wednesday amid mounting criticism from progressives over her work as a corporate defense lawyer.

Alex Oh, who was in private practice for two decades before Gensler announced her new role last week, said in a resignation letter that she was leaving because of a “development” in a case on which she worked as a corporate lawyer. The case involved a class action lawsuit against ExxonMobil, which she represented.

“In light of the time and attention it will take from me, I have reached the conclusion that I cannot address this development without it becoming an unwelcome distraction to the important work of the Division,” she said.

Oh walked away from the job as Gensler has faced growing concern from progressives on Capitol Hill and in the activism community about his decision to hire a long-time corporate lawyer for one of the government’s most powerful posts for overseeing the finance industry.

The episode marked a surprising political backlash from progressives who had cheered Biden’s nomination of Gensler, after the former Goldman Sachs partner emerged as a tough banking regulator when he chaired the Commodity Futures Trading Commission during the Obama administration.

Following Oh’s resignation, Gensler announced that SEC lawyer Melissa Hodgman will return to the role of acting director of the agency’s enforcement division. She had served in the position before Oh’s appointment.

Oh’s departure followed a letter that three leading progressive advocacy groups sent to Gensler on Tuesday saying that they were “surprised and disappointed” by his decision to recruit her. They urged him to withdraw her hiring.

Demand Progress, the Progressive Change Campaign Committee and the Revolving Door Project took aim in their letter at Oh’s two decades of work at law firm Paul, Weiss, Rifkind, Wharton & Garrison, where she represented Fortune 100 companies facing government investigations, with clients including Bank of America and ExxonMobil.

The groups questioned whether Oh “will change her entire legal philosophy toward fully enforcing the very laws and regulations whose enforcement she has built a career of defending against.”

“We therefore ask you to immediately reconsider your decision to name Alex Oh for this position, and instead to select an attorney with a proven track record of public-oriented service, of which there is no shortage,” they said. Oh spent four years working as a federal prosecutor for the Southern District of New York before turning to corporate work.

The complaints from leading progressive groups came as some of Gensler’s allies on Capitol Hill had also begun to question the decision.

“There’s a lot of skepticism that someone who spent two decades helping big corporations dodge the SEC is the person to lead an aggressive revival of SEC enforcement,” said one aide to a progressive Senate Democrat. “A lot of people will be closely watching what she does and now watching Gensler more closely, too.”

The concerns expressed about Oh stem in part from long-running disappointment among progressive groups and lawmakers about lax Wall Street enforcement by the SEC, particularly in the wake of the 2008 global financial crisis.

Former SEC Chair Mary Jo White — an Obama nominee who was U.S. attorney for the Southern District of New York before going into private practice — faced intense criticism from the left, particularly Sen. Elizabeth Warren (D-Mass.), for not doing enough to challenge the finance industry. Oh worked under White during her time as a federal prosecutor.

To be sure, Gensler has aligned with progressives and Wall Street reformers in recruiting some key staffers. His new policy director is Heather Slavkin Corzo, who previously served as director of capital markets policy at the AFL-CIO.

Before Oh’s resignation, some of Gensler’s allies urged critics to give him the benefit of the doubt.

Barbara Roper, the Consumer Federation of America’s director of investor protection, said she was withholding judgment until she could see how he acts as a regulator.

“Ultimately, I trust Gensler to be tough on Wall Street, so I expect his enforcement director will share that goal, regardless of her background,” Roper said before Oh’s resignation was announced. “If that proves not to be the case, we won’t be shy about expressing our views.”