I think the best way to gauge the likelihood of such an outcome is to compare this election to previous ones when pundits speculated that a brokered convention was likely, and then explore how this year is different. Salon’s Steve Kornacki provides a pretty good rundown of previous years here. Let’s walk through each one:
1976 Republicans: The problem that brokered convention proponents ran into that year was that there were effectively only two candidates in the race. As Kornacki notes, while there was some question as to whom the nominee would be, there wasn’t much doubt that either Gerald Ford or Ronald Reagan would receive the nod. Again, the most serious roadblock for a brokered convention in 2012 is that Gingrich, Santorum and Romney all must remain viable. This is possible – again, justpossible — because for now they appear to be developing distinctive bases within the party.
1976 Democrats: The problem here was that Democrats were unsure how to play a prolonged primary race — with the exception of Jimmy Carter. He won nine of the first 10 primaries, which gave him enough cushion to survive late runs by Frank Church and Jerry Brown. Had one of those candidates gotten in earlier, there really might have been a brokered convention. With all three candidates enjoying major wins early in the primary season this year, building such a cushion seems impossible.
1980 Republicans: There was a scenario for a brokered convention that year, with John Anderson playing a spoiler role. But this was never more than barely plausible, as Anderson and George H.W. Bush would inevitably compete for the same types of constituents — neither could live while the other survived. Indeed, that split is probably what enabled Reagan to win Vermont on March 4, and Illinois on March 18. When Bush, rather than Anderson, won Connecticut on March 25, and then Reagan won Wisconsin a week later (again, benefiting from the Anderson/Bush split), Anderson’s Republican candidacy was effectively over. (He later ran as a third-party candidate, of course.)