It is perfectly legal for a political candidate to accept money and/or support from an individual or group and it is perfectly legal for the politician to make and deliver on promises to said individual or group. It is not legal to explicitly state that campaign contributions are the impetus for favorable legislation. So it seems that exchanging campaign contributions for favorable legislation is legal so long as you never say that that is what you are doing.
Obviously there is but a very fine line between legal representation and illegal corruption and there is such a thing as legal corruption.
As I see it, the only solution to the problem of legal and illegal corruption is well informed voters.