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WASHINGTON — Energy Secretary Rick Perry, who has aggressively promoted President Trump’s fossil fuel agenda, is planning to step down by December, ending his run as one of the longest-serving cabinet members in a tumultuous administration, according to two people familiar with his plans.

Mr. Perry’s tenure at the Energy Department has been marked by fierce battles over the role that coal and nuclear energy should continue to play in the United States energy landscape. The agency under his direction has tried and failed to prop up struggling coal and nuclear power plants, and has energetically trumpeted oil and gas development while overseeing plans to reduce funding for wind, solar and other forms of renewable energy.

In recent weeks his role promoting natural gas and coal exports in Ukraine has come under congressional scrutiny amid attention to Mr. Trump’s political actions there.

Two people familiar with his plans, who asked not to be named because they were not authorized to discuss it publicly, said Mr. Perry intends to announce in the coming weeks that he will leave the administration in December. One person said that Mr. Perry had been winding down pet projects, like establishing an office on artificial intelligence, and that his future calendar had been cleared.