The fine line usually placed in the US between corruption and politics as normal is that of the quid pro quo; it's fine to take money from people who happen to share your beliefs, but if you change your votes because of that money, it's corruption.
Politicans have always hidden behind the plausible deniability this gives. You can always claim that you changed your mind on an issue of your own free will, and the donations came later as an unexpected benefit.
But ultimately, it causes well funded interests to dominate political discussion. The teacher's union is almost uncrossable, but how much funding do homeless lobbies get?
Charitably, this is because of survivorship bias - politicans who don't have platforms that generate revenue die out - senators already literally spend half their time raising money [1]. More likely, everyone in congress and the senate sees the writing on the wall and preemtively switches to the right platforms.
The cracks are starting to show in this veneer of innocence, with Chris Dodd earlier this year directly threatening to withhold donations based on SOPA, unless the Obama administration toes the line [2]. It's so long been an open secret in Washington that it was a small step forward for Dodd to take, but it had the unusual effect of exposing the Washingtonian power games to the rest of the country, so it caused quite a bit stir up.
I'm not taking a side on what happened. I genuinely have no opinion one way or another. But if it were as mtgx suggests to curry favour with the MPAA to mend bridges after SOPA, there is zero lattitude on this. It would be corruption.
[1] http://www.huffingtonpost.com/lawrence-lessig/a-big-moment_b...
[2] http://www.techdirt.com/articles/20120120/14472117492/mpaa-d...
Edit: tows->toes. Yes I am an idiot.