Polk is absolutely NOT corrupt - I think you're misunderstanding the meaning of corrupt. Yes, it is the exploitation of power, but not for things like that - generally when you think of corruption, it's meant for things like bribery, political favors, and things like that of an illegal nature. Also, gaining Mexico was not personal gain for James Polk, it was acquiring land for the United States.
For politics in the Gilded Age, I have two hints for you...
1.
Ulysses S. Grant. This sorry excuse for a politician was one of the worst, most corrupt presidents in our country's history. Do some research on him and you'll see what I mean.
2. Read Wikipedia for background information... you'd be amazed
what you can find there. Just don't use Wikipedia as a source, because that's stupid.