The biggest influence be "We the People." Unfortunately the biggest influence is probably "Political Party."
A lot of money is spent entertaining Congressmen to lunches, dinners, etc. There are limitations in the law. Companies have tightened up their giving. They still give a lot but the fact there are so may groups from oil and gas industry and pharmaceutical industry to the NRA and the AARP giving money, that is hot a real issue.
Party allegiance is the main factor because that determines committee assignment, chairmanships, office location and whether or not your bills will ever be heard in committee and may someday on the floor.