A powerful labor union backed a Democratic candidate in a special election last year but never disclosed the amount of money it spent on the effort. The AFL-CIO backed Elizabeth Coppola, a Democrat, in the Assembly District 21 special election last November. Running phone banks out of their office and offering the services of a political coordinator, the union campaigned for Coppola’s election. But not once did the AFL-CIO disclose its political spending backing Coppola. Nor did the Coppola campaign report any AFL-CIO spending as an in-kind campaign contribution. State campaign finance regulations require labor unions and other groups to...