I've suspected for the past couple of cycles, and now believe strongly, that Republicans have become ruthlessly efficient in winding ways to get losing candidates to drop out of the presidential nomination race well before the Iowa caucuses. Tim Pawlenty and Haley Barbour were clear cases of it this cycle; we've had others in previous cycles, including Dan Quayle, Lamar Alexander, Liddy Dole, and Pete Wilson. Then there are less clear cases, such as John Thune and Mitch Daniels, who acted like candidates for a while but pulled the plug before fully committing.
On the other hand, Democrats have been less likely to drop out early. There are some -- Mark Warner and Evan Bayh last time around -- who are in that Thune category, but mostly those who get in stay in until the voters administer the final judgement in Iowa (or a bit later). So in 2008 six plausible nominees (conventionally credentialed and with views of public policy in the mainstream of the party) made it to Iowa on the Democratic side, but this time around it looks as if the Republicans will have just a couple.
So when I see a blog post such as this one from Isaac Chotiner arguing that Pawlenty made a mistake in dropping out after Ames, my initial impulse is to say that it wasn't so much a choice as a consequence of having already lost.
But I have no idea what the mechanism is that separates the Democratic process from the GOP one. I see different results, but I don't really know why. It could even be a random effect, although I'm convinced it's not. It could be that internal party norms are different; it could be that there are specific incentives that are different -- for example, perhaps Republicans (which ones? how?) have either effective threats for those who won't drop out, or promise benefits to those who will.
What we really need is someone to investigate. Hey, reporters! The people who have made these decisions, both the candidates and campaign managers, are out there; wouldn't this make a good story? Actually, I could see it as a good research idea for political scientists (hey, grad students!) too. Why did Tim Pawlenty's story end differently than Bill Richardson's?