I think it's ridiculous to say that a hypothetical conservative third-party candidate would steal a very large portion of Rudy's potential votes if he is the nominee, but that's not what could make the difference. If this person could potentially take five percent of the votes that Rudy would otherwise get, it might help us in, say, Michigan or Pennsylvania, where the race could be close.

The question really comes down to what you hint at: how big of an exception is the Republican base really willing to make. Right now, it's not entirely clear.