I would suggest an Amendment that restricts campaign donations of currency or purchased services to be allowed only by registered voters who are constitutionally recognized as constituents of the office in question. Criminal statutes might then apply to those attempting to mask donations with a"front person". 

   The point is that once money becomes involved in your efforts to further the career of any politician (other than those who constitutionally may represent you)  you are then treading on thin ice.  A candidate's ability and the influence of the constituent voter would again become relevant.