Nate Silver lays out the scenarios under which a brokered convention's odds increase.
1a. Mr. Gingrich leads the delegate count, but does not have more than about 50 percent of delegates.
1b. Mr. Gingrich holds more than 50 percent of delegates but is involved in a significant gaffe or scandal at some point later in the campaign.
2a. Mr. Romney has performed poorly enough in the early states that he is no longer viable.
2b. Mr. Romney is viable but his unfavorability ratings have considerably increased to the point that he no longer qualifies as a consensus choice.
3. A factional candidate like Ron Paul holds 10 or 15 percent of the delegates.
All of these conditions seem plausible based on what we know right now, which is what leads me to believe that a brokered convention is plausible as well.
Except for Chris Christie, I can't see one of the names floated as being plausible or a significantly better candidate.
Jeb Bush would nab conservative intellectuals and establishment support, but one of the giant misconceptions surrounding him is that grassroots conservatives like him. They don't.
If Gingrich were to end up ahead in the delegate count -- only to be replaced by a Bush -- there'd be full-scale revolt among grassroots activists.
Meanwhile, if Mitch Daniels were running, he'd be suffering Tim Pawlenty's fate. Midwestern governor with no charisma. More money, maybe, but less charisma than even Mitt Romney.
Yes, he's got a tremendous, dry sense of humor, but Leno wins America; not Curb Your Enthusiasm. Also, there's his "truce on social issues".
Paul Ryan would be similarly embraced by conservative intellectuals and leaders, but he'd have the massive task of overcoming his youth. He looks about 25 and one of an incumbent's strongest attributes is the fact they've been president for four years, and people are at least somewhat comfortable of letting them run the show.
All these liabilities are tough -- very tough. In fact, they're only dismissible in an academic environment, but the closer you get to people actually voting for a short, quiet Midwestern governor who uses Powerpoints, the harder it becomes.