Republicans probably have a 25% chance
Democrats probably have a 5% chance
If there is a decision made by the convention, we might even have a Republican nominee that isn't even in the race yet (and I'm not talknig about Fred Thompson). It's possible that the convention may draft Mark Sanford to be the nominee. Just speculating.
A deadlocked convention is a possibility, if a remote one. But someone to arise in the last minute at the convention isn't going to happen. The last time that happened and the candidate went onto win was probably Garfield in 1880 (who started off as John Sherman's campaign manager, so to speak, and didn't get a single delegate until at least the third ballot). It certainly hasn't happened since 1924, at the latest. And anyone who got selected at the convention without being in the public eye beforehand would be at a severe disadvantage in organization, name recognition, etc, unless he somehow managed to sweep the nation off of its feet at the convention. The only real possibility for something like this happening would be the Democrats choosing Gore at a deadlocked convention in Denver...and considering how much of a two-man horse race that nomination is looking like, the possibility is extremely remote.
Another reason I think that is unlikely to happen in the future. From a populist perspective, it would be a huge turnoff to independently-minded voters, in that a person that did not receive any votes in caucuses and primaries and yet got placed on a ballot.
The only case I could see it being warranted is if the nominee presumptive died before the convention, and the party's central dictators did not want the guy that finished second.
Me too. I think a split Republican convention would be far more fun than the Democrats just cause the RP normally is so coalesced to one viewpoint and never say anything bad about other party members.
If we had a split convention next year for example, think of what a staunch Midwest conservative would have to say in a speech about Mitt Romney or Rudy Giuliani...
on national TV.
