WASHINGTON — Tom Ridge, the Homeland Security secretary under President George W. Bush, has unexpectedly resigned from the board of Michael R. Bloomberg’s new gun control effort, citing his discomfort with some of the political work the group has been planning.

Mr. Ridge’s affiliation with the group, which Mr. Bloomberg has pledged to give at least $50 million this year alone, was considered a coup for Mr. Bloomberg, the former mayor of New York, because of Mr. Ridge’s Republican credentials.

But Mr. Ridge has notified Mr. Bloomberg’s group, Everytown for Gun Safety, that he could not keep his commitment.

“When I signed on as an adviser to Everytown, I looked forward to a thoughtful and provocative discussion about the toll gun violence takes on Americans,” Mr. Ridge said through a spokesman. “After consultation with Everytown, I have decided that I am uncomfortable with their expected electoral work.”