A top federal official who helped craft regulations aimed at pushing future electric vehicle (EV) use is departing the Biden administration in an unexpected move, according to an internal agency memo obtained by Fox News Digital.

Ann Carlson — who is currently the acting administrator for the Department of Tranportation's National Highway Traffic Safety Administration (NHTSA) — will step down from her role on Dec. 26, 2023, and depart the agency entirely at the end of January. Patrick Lally, who heads NHTSA's governmental affairs office, made the announcement in a memo circulated among agency staff late Friday.

"Ann has been a tremendous leader — her time as Acting Administrator of NHTSA was time limited under the Vacancies Reform Act," a DOT spokesperson told Fox News Digital in a statement. "She is returning to her previous role of senior advisor and Chief Counsel for one month to assist with the transition."

"Under her leadership, NHTSA strengthened child seat standards, moved to require Automated Emergency Braking in light and heavy vehicles, oversaw major recalls, held registered importers and those who commit odometer fraud accountable for their bad acts, and after horrific increases in previous years, for six straight quarters roadway fatalities have declined," the statement continued.

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President Biden nominated Ann Carlson to lead the National Highway Traffic Safety Administration in February 2023.

President Biden nominated Ann Carlson to lead the National Highway Traffic Safety Administration in February 2023. Her nomination failed in May, but she continued to lead the agency, including crafting the most aggressive national fuel economy standards ever crafted. (Ting Shen/Bloomberg via Getty Images | National Highway Traffic Safety Administration)

Carlson, though, had garnered strong opposition from Republicans over her past climate activism, support for regulations targeting gas-powered vehicles and how she continued to lead NHTSA in an acting role even after failing Senate confirmation for the post in a permanent capacity.

In January 2021, the Biden-Harris transition team hired Carlson, then an environmental law professor at UCLA, to serve as NHTSA's chief counsel. While the position didn't require Senate confirmation, Carlson oversaw key agency initiatives in that role and began serving as acting administrator in September 2022.

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Then in February 2023, Biden nominated Carlson to permanently lead NHTSA. In the months that followed, she faced heavy opposition from Senate Commerce Committee Republicans for her past work in the private sector advising plaintiffs on climate litigation and comments she made via email about her role with the Biden administration.

Her nomination was then suddenly withdrawn in May, but she remained acting administrator, sparking fury from Republicans and energy advocates. And after her nomination was withdrawn, Carlson crafted new fuel economy regulations designed to incentivize greater EV purchases.

Ted Cruz during Senate hearing

"As Senate Republicans pointed out repeatedly, Ann Carlson was serving as acting administrator contrary to the law," Commerce Committee Ranking Member Ted Cruz, R-Texas, told Fox News Digital. (Tom Williams/CQ-Roll Call, Inc via Getty Images)

"As Senate Republicans pointed out repeatedly, Ann Carlson was serving as acting administrator contrary to the law," Commerce Committee Ranking Member Ted Cruz, R-Texas, told Fox News Digital. "Her attempts to hijack this safety agency by rewriting auto fuel economy standards into an unpopular de facto EV mandate should not stand as a result of her illegal appointment."

"I am hopeful President Biden will nominate someone with the requisite safety experience to lead NHTSA, not another radical, ‘Green New Deal’ activist," he continued.

In addition to leading opposition to Carlson's nomination for NHTSA administrator, Cruz introduced a resolution alongside Sen. Cynthia Lummis, R-Wyo., in September which would have effectively defunded Carlson. 

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Cruz has argued that Carlson maintaining her position as acting administrator violated the Federal Vacancies Reform Act which prohibits "any person who has been nominated to fill any vacant office from performing that office’s duties in an acting capacity." He noted the provision and the fact that Carlson did not serve in the position of first assistant to the previous NHTSA chief for more than 90 days before he resigned, precluded her from her current position.

"President Biden’s illegal appointment of Ann Carlson, who was effectively rejected by the Senate due to concerns about her extreme agenda and radical record, will only yield more mandates, higher costs for families and a less vibrant economy," Cruz said in September.