The main reason that a brokered convention won't happen is that Rick Santorum is a terrible candidate. In the past few days, he has questioned President Obama's "theology" saying it is "not a theology based on the Bible, he has given a biblical basis for disbelieving global warming, he has come out against prenatal testing (amniocentesis), and an old speech from 2008 resurfaced in which Santorum says Satan is attacking the US and mainline American Protestantism is "in shambles". The 2008 speech was promoted heavily yesterday by the pro-Romney Drudge Report and represents the conservative establishment dogs coming out against Santorum like they did with Newt after he won in South Carolina. Unless Santorum can reverse the recent tide in Michigan that's seen Romney recover, he likely won't be able to stay with Romney over the long haul.
Another problem with a "white knight" like Jeb Bush or Mitch Daniels saving the party at a brokered convention is that a nominee from such a convention would be starting from a weak position. Why would Daniels or Bush want to get into this race when they can keep their powder dry for 2016, when they can run a full campaign and not have to face an incumbent. A "prominent Republican senator" told ABCNews that if Romney loses Michigan, "we need a new candidate". But unless they think Sarah Palin's better than Romney, they'll probably have to hold their noses and nominate him.
In other news, there's a big debate tonight in Arizona. It's the last one before Super Tuesday on March 6th. It's Newt Gingrich's last chance to make a splash, and a good moment one way or the other could decide between Romney and Santorum in Michigan. Expect a post on the debate either tonight or tomorrow.