This, from Gary Wills at the New Yorker, is one the most important realities of contemporary American politics:
[T]he man being voted for, no matter what he says, dances with the party that brought him, dependent on its support, resources, and clientele. That is why one should always vote on the party, instead of the candidate. The party has some continuity of commitment, no matter how compromised. What you are really voting for is the party’s constituency. That will determine priorities when it comes to appointments, legislative pressure, and things like nominating Supreme Court justices.
Even among political reporters, there’s a tendency to separate the candidate from the party, as if a president is somehow separate from the constituencies that he represents. But the truth of the matter, as Wills points out, is that in most instances, the president works to fulfill the priorities and demands of the groups who elected him. Mitt Romney may or may not be a moderate—it doesn’t matter. What matter’s is that—if elected president—he’ll represent a Republican Party that has abandoned moderation in favor of radical cuts to the size and scope of government, and regressive views on social issues.
If you want, you can play this game with Barack Obama circa 2008. Anyone who looked at the Democratic coalition at the time, and thought Obama wouldn’t try to pursue health care reform, or support our involvement in Afghanistan, is fooling themselves.