I'd like to add a litte more along these lines:
As suggested, taking a look at Opensecrets shows that big money in politics does come, in a large part, from unions:
http://www.opensecrets.org/orgs/list.php
So, a lot of big money in politics comes from unions and goes mainly to the Democratic party (at least the top 15 or so), which may be contrary to the only big corporation and Republicans thing most of us expect (especially from all those Obama campaign emails I get about "grassroots").
What the big unions do is strongarm you as a young person into becoming a VIP member (whose fee is eligible for political contributions) and then don't give you a say. Also, when layoffs happen, you're the first to go since the
ONLY thing that matters is seniority. Sadly, when it comes to many things, the big unions don't look too different from the big businesses.