Christopher Oechsli, a lawyer who previously was an aide to former Democratic Sen. Russ Feingold, will lead Atlantic Philanthropies until a new president is named, the board of the grant maker, which is worth about $2-billion, announced today.

The announcement came as leaders of progressive groups expressed concern to the foundation’s board about Atlantic’s future direction.

Mr. Oechsli is not new to Atlantic, which is based in Bermuda but has a New York headquarters: He first joined the foundation in 2000, serving for three years as its program director in Australia, Cuba, and Vietnam. He then oversaw grants on population and health until 2005.

He will take over on an interim basis from Gara LaMarche, who has led the grant maker since 2007. Mr. LaMarche announced this month that he would not seek a second, five-year term and would step down in September.

A former American Civil Liberties Union official who has sharpened the foundation’s focus on social-justice issues, Mr. LaMarche said he was leaving in part because Atlantic planned to focus more giving on biomedical research and higher education in the years before it closes its doors in 2016.

The announcement set off concerns among progressive nonprofit leaders, some of whom sent a letter to Atlantic’s board emphasizing the foundation’s important role in supporting public-policy advocacy.

“It’s like a meteor has hit,” said Tom Sheridan, a veteran lobbyist for nonprofit groups.

Atlantic’s board has been meeting since Sunday to discuss the leadership transition and the foundation’s future.

“We received the letter and the board is taking it very seriously,” said Ben Wyskida, a spokesman for Atlantic. “We’ll be in a much better position to respond after the board meeting concludes tomorrow.”

Suzanne Perry contributed to this article.