Professor Anthony Davies at Duquesne University made a series of charts about political giving. Here is a very interesting one:
It breaks down the top 100 donors to political candidates and parties from 1989 to 2009. As we can see see, labor unions are the biggest donors by far. During that period, they gave almost $500 million to political parties or candidates (the second biggest donor is the investment industry with a little less than $200 million). Also, most of unions’ donations go to the Democratic party or Democratic candidates. More on unions here and here.