Never happen BEFORE the elections. And Obama will point to it as Romney being on the side of ObamaCare - an argument his media minions will gleefully spread far and wide.
IMHO, we need to push for a brokered convention. Even if it doesn't happen, we need to take the fight, long and hard, to RINO elitists within the Party.
It would be Romney's only chance in November. Super Deligates will keep a brokered convention from happening.