Over the past two decades I have defended Republican candidates I couldn't stand based on the logic that a two-party system is often about picking the least terrible candidate. I tried to reason with FReepers that a failure to vote for, say Mitt Romeny was mathematically equivalent to voting for Obama.
With that being said, should Rand Paul win the nomination, I will find it very hard to vote for him, even if he is running against Hillary Clinton. It is actually possible that her foreign policy choices might be better than Paul's.