He is expected to arrange government blessings for political contributors, within the constraints of the law and ethics. Traffic moves both ways along these arteries. The beneficiaires are supposed to be grateful and show their appreciation. This sometimes takes the forms of campaign contributions. It's a clumsy system that has been distorted by the special interests.
Money can be decisive in close elections.
Vested interests are ready to funnel lots of money to selected campaigns. When campaign financing is offered by a pressure group, the politician must either sell out on the dotted line or alienate this powerful bloc. Such corruption is both routine and bi-partisan. More often than not, it produces scandals which turn off more voters, and the corruptive circle keeps widening. It is still true, incredibly, that our elected representatives want to do what's right, and try to vote their consciences.