The latest tax documents available show combined income of American Federation of State, County and Municipal Employees (AFSCME) dropped 45 percent in 2012 - the first full year of the law, according to The Capital Times.
In 2011, the four councils that make up the state organization reported a combined income of $14.9 million. In 2012 that dropped to $8.3 million. Dues revenue dropped 40 percent to $7.1 million.
Walker and supporters of the law said it was a way to help local governments reduce the costs of employee benefits, but the legislation also included measures aimed at financially weakening unions by ending automatic dues deductions.
The union's Council 40 executive director, Rick Badger, says that while the declines in revenue stemming from the law were expected, he has been encouraged by the number of workers who have continued to pay voluntary dues.
"In fact, what (the law's) architects might find surprising is our resilience," he wrote in an email. He said thousands of "front-line workers are remaining engaged in fighting for their rights despite heavy-handed political attempts to silence them."
While public unions no longer enjoy the official bargaining power that they exercised in recent decades, he said many public workers continue to value their presence as advocates for their rights and welfare.
AFSCME is second only to the Wisconsin Education Association Council, or WEAC, in members in Wisconsin. It has long been a powerful player in state politics, funneling money directly to campaigns and running independent television ads in support of pro-labor candidates, as well as providing a legion of employees and member volunteers who made sure their union brethren voted on Election Day.
The law has also hit other big unions in the state. For instance, WEAC, the state's largest teachers union, saw its revenue drop from $26 million in 2011 to $20 million in 2012.