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.