Loc: Port Angeles, WA
I would suggest your study on opensecrets.org There you will find where all the politicians get their money. To simply say its corporations would be wrong. Those giving/taking/running are a diverse group of whatevers. Individuals and corporations and NGO's, etc.
My own thought is that the problem is not so much the bribery and corruption but the politician him/her self. Some are stalwart and determined to do the job and others consider the job to get as much as fast as they can and then move on to 'senior' status of the perceived wise.
Lobbying is how our leaders formalized bribery and corruption, as far as I am concerned. I have always found that pretty interesting. If an elected, however, decides to go outside of this system they do get crucified big time by their colleagues. That doesn't happen often, but it does happen. I have also found how the elected deal with the treasure is also interesting. I think it usually ends up in some kind of non-profit trust run by family and supports said family for years.