While Vito was incredibly successful in organized crime, he realized there was even more opportunity in the becoming the pezzonovante - the one holding the strings. In the scene before his death, Vito wistfully tells Michael that he never wanted this life for him, and rather Michael were "Senator Corleone" or "Governor Corleone."
I don't have the novel handy, but at least in the film, Vito held Senators in higher esteem than Congressmen. When delegating the Enzo immigration favor, Vito wanted it to go to a particular Congressman. But it was a Senator that was actually invited to his daughters wedding; Tom mentions the judges too, but nary a Congressman.
That's a subtle, but important distinction that I had not considered. You can even hear the respectful tone in Vito's inflection when he said "Senator Corleone." In the novel, Vito disparages judges who "sell themselves out" like the worst whores on the street (or something to that effect). I think this was when he was talking to Bonasera. Later in GFIII, Michael makes quick mention how its always good to have judges friendly to the Corleone's interests.
