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.