There a couple of points in this. First is that symantic hair splitting over is a tax is not a tax is completely unimportant. People's feelings on Obacare are already set, and calling this a tax isn't changing anything. What may have some effect though, is how the candidates present themselves on the issue when people are listening. For Obama that's easy, he gets to call it hair splitting, say that he still believes it's a penalty not a tax, and even if it's a tax it's still the right thing to do. But for Romney, he's trying to use it as a cudgel that only hits his opponent, he's trying to say, "Hey I did it right in MA, but it's wrong nationally." So he gets into these logical twisters, that don't hold up under any kind of sustained questioning.