A Question for Jim

Seriously, I'm thinking about the whole comically-corrupt-and-delusional governor thing, and I have one point that I am unclear on:

What is the distinction, legally, between horse-trading politics and corruption/influence-peddling?

I was thinking about Blago's demand that the Trib change its editorial tone in order for Blago to approve the sale of Wrigley (or some such thing). That is what struck me as sorta hard-ball politics but not obviously corrupt/illegal.

Certainly, asking for campaign dollars or a job for yourself or a family member for political favors is corrupt. That's clear. I remember, though, some horse-trading that took place before Medicare Part D was approved, where one GOP rep finally signed on when his son was promised electoral support from GOPAC, which was criticized but not investigated. Then there are all the "you support my bill-I'll support your pet cause" quid pro quo's that are exchanged with little to no controversy. And then there was the not-so-tacit understandings that Tom Delay put out on K street that only lobbyists "in the book" of GOPAC donors could expect favorable treatment in the House.

So, where's the line? Legally, Jim, is there a commonly-accepted definition of what sort of political leverage is ethical and acceptable, and what is not? (And I'm not asking what is "provably" corrupt, but what would be defined as corrupt if fully understood.)