In their attempt to enjoin Tobacco money, political contributions, lobbyists and legislation, Common Cause of Minnesota used a meat cleaver instead of a scalpel. This is a pittance in politics today. That is not to say that they are completely wrong in attempting to link contributions with legislation. Every legislator has a constituency and a vast array of issues that they are sympathetic to. Those interests and their supporters are where one would find a smoking gun.
Republicans receive contributions from business interests and anti-government groups.
That is how a person can differentiate between the two parties. One supports management and the other supports the workers. One wants to consume environmental resources the other seeks to protect them. This may be a broad generalization, but mainly it holds up. The system of politics is run by money, it's a necessity.