Top political donors - Labor Unions.

Breakdown by industry for the last 20 years shows labor unions give more than twice as much as the next biggest group - the investment industry.  Big oil, airlines and even lawyers are way down the list.  Party and individual donations are aggregated in the chart.  That union money is about the same sum that just went down the toilet for Obama's green pet, Solyndra.

The first chart gives selected industries with party breakdown. (Union total doesn't match cleanly with numbers in the second chart.)

The second shows the 29 largest donor industries, no party breakdown. (The rest of the top 100 aren't displayed on this frame).

The charts were prepared by Antony Davies of Duquesne University by querying the Open Secrets database.  (h/t Ace of Spades and De Rugy)  The Democrat party appears to have been bought by Big Labor.  Although lawyers donate almost every penny to the Democrats, and are way overrepresented in Congress, the number of Republican lawyers is not much less than the number of Democrat lawyers in public office. (60% of the senate and 37% of the House of Representatives are lawyers with party affiliation of 81R and 123D. Lawyers are about 0.6% of the US workforce.