This based on the 2008 elections. The corporate rate was very similar to the general election vote for Obama vs. McCain, and coporation favored McCain more than the general populous. I'm at work in between meetings so I unfortunately cannot find the source, but I recall that the combined wealth of all the unions in the US would place them about 200 or so on the Fortune 500, that means there are 200 corporations with more money than every single union combined, comparing the two is comparing apples and oranges.
Here is a list of the top donors for the 2008 election-
http://www.opensecrets.org/bigpicture/topcontribs.php?cycle=2008
Your argument is a non-sequitur- Regardless of where any unions might place on the Fortune 500 list, what matters is how much money they donate, and who they donate it to.
Note that there are unions at slots 8,20,26,29,30,32,38,41,42,44,45,and 51. That's at least 12 different unions among the top 51 donors.
And note who the top donors (business or unions) give money to- predominately the Democrats-
23 of the top 50 are listed as strongly or solidly Democratic, while only
1 of the top 50 donors is listed as either solidly or strongly Republican.
45 of the top 100 are Democratic, while only 4 are Republican.
Again, I am fine with any restrictions placed on donations. But lets apply them equally, and it should include union donations, too.