Just four months after the AFL-CIO announced the formation of its super PAC, Workers' Voice, the group has emerged as a major force, ranking No. 10 among super PACs in terms of money collected as of the end of July, with more than $7 million already spent, and the backing of the House of Labor's treasury.

However, the union super PAC weathered a major disappointment in its first outing: It was formed as part of an unsuccessful effort to unseat Gov. Scott Walker, R-Wisc., in a June 2012 recall election.

Since its inception, Workers’ Voice has branded itself the ...