Writing in Slate, Jacob Weisberg is notably more skeptical about a third-party presidential bid than his elite pundit colleagues:

It is a rather obvious point that leaving the country’s biggest problems to fester can’t be good policy. What is less obvious is that it may not be good politics either. A two-party system is a zero-sum game, in which Republican gain ought to mean Democratic loss, and vice-versa. But because the politics of blockage, blame, and stagnation tends to breed disgust with both sides, it can pave the way for big anti-incumbent swings and third-party movements. Both John Anderson in 1980 and Ross Perot in 1992 ran on their ability to break through the stalemate in Washington. Because of the strong and varied presidential field, a significant third-party bid seems less likely in 2008; John McCain and Barack Obama both owe their popularity to a reputation for speaking more plainly than other politicians. But if the logjam persists, it’s not impossible that a Michael Bloomberg or someone else could wage an independent candidacy on the argument that neither Republicans nor Democrats are leveling with the country.