Sonny: Santino is the most passionate of the Corleone brothers and gets very worked up about politics. He's well informed, which would make his views convincing if he weren't so hotheaded. Instead, his quickness to anger makes him very vulnerable. Sollozzo says of Santino "You can't talk business with him." You can try to talk politics with a Sonny voter, but you won't convince him of anything.
Michael: Michael might know who he's voting for, but there's no way you do unless you're Tom Hagen, the only person Michael trusts. Michael is a cold blooded realist who won't vote on one issue but will carefully weigh the pros and cons of each candidate. He's not concerned about making the moral vote; he'll vote for the guy whose policies will help him personally. While he has no interest in bipartisanship, he can feign one pretty well. He keeps his true views to himself while convincing both Republicans and Democrats that he believes their version of the story. Just as he did with Frank Pentangeli and Hyman Roth, he'll hear how you defend your story and use that to make a decision. Michael is the most respected and feared among the brothers. Politicians, even the ones who disapprove of him such as Senator Geary, work hard for his approval.
Tom Hagen: Tom is much more idealistic than Michael and wants to see the two parties working together to improve the country. He's very smart and extremely well versed in the law, which makes his opinions highly valuable. He's not spineless and is more than willing to stand up to a Sonny voter. There is a great level of mutual respect and trust between Tom Hagen and Michael, but Michael holds all the power, and Tom will ultimately vote for whomever Michael chooses.