They're not free marketers who happen to take bribes on occassion. The policymaking is fully continuous with the corruption.Both regular readers of my blog will recall that I've written about this before. Personal corruption makes good TV, but it's not really the point. Proof of personal corruption wouldn't make DeLay bad for the country. He'd be bad for the country even if he were Mr. Clean. As long as he was on their side, he'd still get money from corporations defending their interests. If he wasn't, he wouldn't get much no matter how corrupt he was.
Those looking to purchase an agenda probably prefer to buy from the less corrupt. True believers make better spokesmen. They're more reliable. They don't end up in court or hauled before ethics committees. They don't drag their donors' names through the mud with them.
Even without petty corruption, candidates who accept large donations are more likely to represent the interests of their donors than not. Donors support candidates because they expect them to advance an agenda, and they tend support agendas that advance their interests. A candidate's donor list tells you more about what that candidate will do than months of press reports, and the longer a politician's career, the more predictive the donor list will be.