Recent Labor Department filings show just how drastic this decline has been for organized labor.
American Federation of State, County and Municipal Employees Leadership Council 40, based in Madison, reported it had just under 32,000 members as recently as 2011, when Walker introduced his reforms. In a May filing, membership had dwindled to a little more than 13,000, a loss of about 18,000 members in four years.
Earlier this year, Milwaukee-based AFSCME Leadership Council 48 reported that its membership was 3,400, down from the 9,000 it had in 2011.
The unions have taken financial hits as well. Council 40 reported taking in $5.5 million in annual membership dues in the May report, down from the almost $10 million it reported in 2011. Council 48 reported taking in $730,000, down from $2 million in 2011.
Walker's 2011 reforms ended the state's practice of automatically deducting membership dues from public sector workers on behalf of their union. It also allowed those workers to opt out of being union members, required the unions to submit to annual recertification votes and limited collective bargaining to just wages.
The membership declines suggest that thousands of workers have simply walked away from their unions, either because they did not want to be members in the first place or because the reforms sharply limited what the unions could do for them even if they stayed.
The governor said the reforms were needed to wrest control away from unions in order to get control of the state budget. They were also needed to give individual workers more choice.
State union leaders and Democrats fought doggedly against the reforms, arguing they were a partisan effort to undermine labor organization. Activists clogged the state capitol for weeks, Democratic legislators fled the state in an effort stall the reforms and Walker had to fight off a recall election in 2012.
The Wisconsin Supreme Court effectively ended legal challenges to the reforms late last month. Walker has argued they could still be undone if Democrats unseat him in the fall election.
Polls show he is running neck-and-neck with Mary Burke, the likely Democratic challenger. Burke has indicated she will not run against the reforms, saying it is not a winning issue for Democrats.
Read the Washington Examiner's April interview with Scott Walker, in which he discussed the reforms and the struggle to enact them.