So seven of the Supreme Court Justices, including all of the ones who voted for corporate free speech in Citizens United, have decided that unions aren't the same as corporations and don't have the same political rights.
The court ruled 7-2 that unions can't use their members' dues for political campaigns unless they ask first. That doesn't sound so awful; gee, if you're going to take my money and spend it on a candidate I don't like, shouldn't I get a chance to say no? (Of course, that's already the case, and this ruling is pretty narrow -- the union wanted to raise the money and offer refunds to members who asked. The court says you have to ask first.)
But the distinction here is interesting. Corporations don't have to ask their shareholders in advance before they donate money to political campaigns. In fact, they don't have to ask shareholders -- who, in theory, are the members of the corporation, the owners, the ones whose financial interests are most directly at stake -- at all.
Pacific Gas and Electric Co. can use millions of dollars of its shareholders' money to support candidates and causes -- and if you're one of the poor retired workers who holds PG&E stock as part of your pension, you think you have any say? No, you don't.
So what this does is further erode the power of the one large established group that can sometimes spend close to what big business does, and that's organized labor.