While it is true that the direct donation of money is illegal, the running of political ads for or against a candidate is legal in the US, no matter how much is spent. I don't see any moral difference between "I will give you $100k to stop this regulation." and "If you stop this regulation, I will donate $100k to a PAC that supports your reelection." Both are, in my opinion, complete bribery and abuse of an office, but the latter is completely legal.
Yeah, it'd be great to shut down that UTC plant that subsists entirely on defense contracts, but it's been in the community for 50 years and provides thousands of jobs. Good luck running for that district on a platform of wanting to kill local jobs.