When you raise money from donors to win election to an office, those donors effectively become your constituency, rather than the people living in your state or district. Those donors will also offer you strangely sumptuous speaking fees or a lucrative lobbying position after you leave office.

If politicians' incentives can be aligned with their constitutional constituencies (not donors), they can make decisions that are actually in the best interest of those they represent, rather than the highest bidder.

Are you saying it is simply having some power over the budget that corrupts politicians, and not what they have to do in order to obtain that power that corrupts them?