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In a cryptic message Friday, Sierra Club Executive Director Michael Brune said he plans to step down, effective at the end of this year.

Brune’s decision was viewed as a blow to the U.S. environmental movement and green activists in the Democratic Party, Politico reported.

In a statement, Brune apologized "for any instance" in which employees or associates of the Sierra Club did not "feel safe, supported and valued" – but he cited no specific complaints or allegations, according to the news outlet.

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In July 2020, not long after the death of George Floyd in police custody in Minneapolis sparked a wave of riots in cities across the U.S., the Sierra Club announced that it planned to address alleged racism and White supremacist sentiments within the organization across its history of nearly 130 years.

Brune used a brief Twitter message about his decision to take a stab at Donald Trump, referring to him as a "toxic President."

Sierra Club Executive Director Michael Brune speaks in San Francisco, May 18, 2017. (Getty Images)

Sierra Club Executive Director Michael Brune speaks in San Francisco, May 18, 2017. (Getty Images)

Brune wrote last year that Sierra Club founder John Muir, who died in 1914, and others were linked to offensive views – but on Friday, Brune did not address whether any more recent incidents had prompted his own decision to step down.

In June, the Sierra Club called for the U.S. to make reparations to Black Americans as part of an effort to address historical wrongs, Politico reported.

The Sierra Club has long tried to wield influence in Washington. Recently, it expressed opposition to the re-nomination of Jerome Powell to head the Federal Reserve Board, viewing him as not aggressive enough in advancing climate policies the club prefers, Bloomberg News reported.

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The club has also expressed concerns about whether President Biden would sufficiently pressure auto makers regarding tougher emissions standards, the outlet reported.

Until a successor for Brune is named, Sierra Club Foundation Executive Director Dan Chu will serve as acting executive director of the club while organizer Eva Hernandez-Simmons will serve as managing director, Politico reported.