As ridiculous as I think the Republican Establishment is, I cannot for the life of me believe that they would be STUPID enough to bring in a COMPLETELY DIFFERENT candidate at the Convention. If they brought in someone that had not received SINGLE BALLOT VOTE in the primary, all hell would break loose. My bet is that the pressure would be HEAVY for Romney unless he drops out before then.
The delegates are going to be able to vote for whoever they want if the first vote fails, and Ron Paul delegates are trying to get signed up to represent Newt, Rick and Mitt at the caucuses. Each state still gets to select from that pool when sending to the national convention, so I suspect establishment types will be able to influence the delegate selection with as many of their loyalists as they can. I think Paulites are most likely to vote for Romney, because they are probably really turned off by the social conservatism and anti-drug policies of Newt and Rick.
I think we can expect Newt and Rick’s delegates to go for each other, especially since I would think either one of them would agree to be each other’s Vice President (hopefully the delegates could be made aware of this during the process). So I would say unless the combined delegates of Newt and Rick would make a 2/3rds majority, enough to make up for planted cronies from Ron Paul or the establishment, either of them are unlikely to win a brokered convention.
The other wild card, is that Mitt will probably be trying very hard to get Rick to be his Vice President and get his delegates. I think Mitt is keenly aware that he has to sign on a strong social conservative as his V.P. to shore up the base. Unfortunately, I don’t have enough confidence in Rick to say that he would not make that deal. Newt might have to be ready to allow Rick to take the Presidential slot and settle for V.P. to sway Rick away from Mitt.