When Mayors Against Illegal Guns Action Fund, New York Mayor Michael Bloomberg's vehicle for pushing for tighter gun control regulation, ran ads Sunday morning calling on President Barack Obama and his Republican opponent, former Massachusetts Gov. Mitt Romney, to offer plans to reduce gun violence, they became the first dark money group to publicly disclose its donors.

Bloomberg's group, a social welfare organization set up under section 501(c)4 of the tax code, is the first to comply with a U.S. District Court ruling requiring such organizations, which normally keep their donors secret, to disclose them ...