Hendrik Hertzberg's succinct, eloquent summary in The New Yorker of the issues at stake in Wisconsin includes an eye-opening paragraph (emphasis mine):
Of the five biggest non-party organizational contributors to political campaigns in 2008, the top two were unions, both of them pro-Democratic and both composed partly or wholly of public-sector workers. The other three were pro-Republican business groups or PACs. In 2010, after the Supreme Court threw open the cash sluices in the Citizens United case, only one union made it into the top five, and it came in fifth. And from now on, thanks to five Justices, corporate campaign spending will be literally limitless.
It's almost too neat. Corporate money floods into the election process, Republicans win a huge victory, and a crackdown on unions starts everywhere.