The thing to remember is that Bill Clinton only won by splitting the GOP vote. If Rudy causes a third-party candidate, Hillary wins in a "landslide" with 43% of the vote, taking states that she could never otherwise take.
Sorry but No Republican or Social Conservative would be stupid enough to run third party. They know it's a lose lose situation if they do that and would serve no purpose but to nominate a Democrat into office. It just won't happen. Even Ron Paul is running as a Republican. He's not stupid enough to run for third party either. The threat of a third party candidate is an empty threat and will be an empty threat in 2008. The only one thinking of possibly forming a third party is a hollywierd star called Sam Waterston (from Law and Order Fame) who wants to form a liberal third party.