There is an interview with historian Garry Wills, in the New York Times today. He was asked whom he would vote for. His answer " I always vote the party. It's ridiculous not to. You may like a person who is Republican, but if you vote for that person, you're voting for all the apparatus that comes along with it."
I find a flaw in this way of thinking. If you find a candidate whom you like better than your party's candidate, who is more appealing to you, or better expresses your points of view, even if that candidate belongs to a different party than you do, you should be open to voting for that person.
Lincoln joined the newly formed Republican party because the Southern Democrats were so staunchly pro slavery and would resist any moves to amend it. We would not have a Scott Brown (a Republican with an open mind) in the Senate now if people from all party's didn't vote for him.
There is a time when voting the party may have a stronger pull. If you think that candidate will shift the balance of power in the particular chamber, then a true party member may have to reconsider their vote.