First, a mea culpa: I'm several weeks late to this story, but it's so significant that I figured I'd adopt a "better late than never" approach and bring this to your attention anyway:
Wisconsin membership in the American Federation of State, County and Municipal Employees - the state's second-largest public-sector union after the National Education Association, which represents teachers - fell to 28,745 in February from 62,818 in March 2011, according to a person who has viewed Afscme's figures. A spokesman for Afscme declined to comment. Much of that decline came from Afscme Council 24, which represents Wisconsin state workers, whose membership plunged by two-thirds to 7,100 from 22,300 last year.
A provision of the Walker law that eliminated automatic dues collection hurt union membership. When a public-sector contract expires the state now stops collecting dues from the affected workers' paychecks unless they say they want the dues taken out, said Peter Davis, general counsel of the Wisconsin Employment Relations Commission. In many cases, Afscme dropped members from its rolls after it failed to get them to affirm they want dues collected, said a labor official familiar with Afscme's figures. In a smaller number of cases, membership losses were due to worker layoffs.