Donate the maximum amount legally allowed (as an individual).
Tell the Member that you would like to become a bundler.
(A bundler is a person or small group of people who pool or aggregate contributions "from the community" and then deliver them in one lump sum to a political campaign).
Once you have raised a sizeable amount, deliver the money to the Senator so they can use it wisely. In turn, trade stock options based on insider information to the tune of millions, be super glad that you helped the democratic process!
Even if both were ilegal the democratic political system is still corruptible. Let's say you want to make a law that's going to help a lot of people, you still need others from the opposing party to vote for it, (even if there are more than 2 parties) so they also need you to vote in something for them. Even if there is no money in the middle, it is still a corrupt system. Is there a better one today? I don't think so. But still crap
EDIT: in advance of questions, to explain the difference, what you have described is called political compromise (finding solution that suits both parties). We need MORE of that. Corruption is when politician personally benefit from "selling" their votes.
The stated problem is actually more pronounced with a larger number of parties than 2. If the governing party is strong, but doesn't have majority, and the opposition is split, they usually need to trade favours with the opposition parties to get anything done.