I guess you could say lobbying legalizes a portion of what is understood as corruption. (Not everything, obviously.) Under the guise of "defending special interests", or something similar.

You shouldn't be allowed to finance people whose decision you stand to profit from.

I guess the only "simple" way to solve this is to outlaw the private financing of politicians and their campaign. It should all be public money. Not that easy to operationalize (almost nothing of interest is), but feasible, desirable and fair I reckon.