A "bulldog" Ohio lawyer is helping conservative groups influence elections as opaquely as legally possible. WEST CHESTER, Ohio—Just outside Cincinnati, tucked among insurance agencies, hair salons and a yoga studio, is the nexus of one of the nation’s most mysterious networks pouring secret money into elections. “Langdon Law LLC Political, Election Nonprofit and Constitutional Law,” reads its small sign, which faces the building’s parking lot rather than the street. On a Tuesday afternoon last month, that parking lot was empty. No one answered the Langdon Law office door. Phone calls went unreturned. Unlike other heavy-hitting political lawyers, David Langdon doesn’t...