To add on a few additional points of evidence: 1) In the most recent set of national elections, there was not much correlation between money spent and probability of victory [1]. While second-order effects may still be problematic, I think it's worth reemphasizing.
2)Congressmen doing things that are against the will of their constituents leaves them massively open to primary challenges. See what's happening with Chambliss or other republicans in the recent budgetary crisis.
3)I do think there is a particularly pernicious form of corruption in Washington, stemming from the fact that lobbyists and other non-governmental entities often draft a large number of bills. This doesn't manifest as a specific national policy goal not being achieved, but rather causes subtle aims to be achieved in quiet fashions. This is not a problem of money, though -- it's simply a function of availability, and the convenience factor of who sees whom on a daily basis. Perhaps this could be fixed by empower congressmen and senators to carry larger staffs (staves?) than they do today?
[1] http://sunlightfoundation.com/blog/2012/11/09/how-much-did-m...