But there are others who believe that a (D) is short-hand for something else, namely that the politician generally hews a bit more closely to a particular set of policies that have worked a little better than the policies adhered to by people who have an (R) after their name. Its not a guarantee of performance.
I have noticed that many people who take the second approach have been criticizing Obama's approach even before he took office. I for one have posted (over and over and over) on the folly of bailing out the financial institutions that have caused this mess (since when is taking from the poor and middle class and sending to the fabulously wealthy compatible with the policies that generally define Democrats?), and I've had a few posts noting that Christina Romer was a very poor choice for CEA chair (Mankiw's endorsement alone should have been a tip-off even to someone who never read her "narrative economic history" paper).
Sure, exogenous factors do matter. Truman was a Democrat, but the economy was awful for the first few years of his administration; transitioning from WW2 to a peacetime economy ain't easy. And it sucked in 1933 too, which was FDR's first year. This despite the fact that annualized, real GDP per capita grew much faster under FDR than any other President over the time period for which the BEA computed GDP, even leaving out the War years. Which also indicates something else - the economy might still turn out alright despite the many Obama mis-steps. Time will tell.
But there are a few other things - for one, you don't have only one (R) or (D) administration to look at. You have probability and statistics. (And to forestall the point that inevitably gets brought up again and again and again and again, that's what degrees of freedom are for.)
And while no theory is perfect, the better ones require fewer exceptions. If Obama starts acting like a Democrat and the economy still sucks by the end of his term, that means Democrats have to explain away Obama and Truman. Republicans still have to explain why Republicans generally do so much worse when it comes to real GDP per capita growth rates. The BEA has only been calculating real GDP per capita since 1929, and yet the best performing Republican, Reagan, is beat out by four Democrats, and all but one of the entire bottom half of the sample is taken up by Republicans. Tip to Megan McArdle: nobody can tell you for sure who's going to win next year's World Cup in South Africa, but betting on Grenada to pull it off is a bad idea.
And then there's the question that McArdle herself keeps bringing up - the mechanism. The mechanism doesn't apply if you don't have the right inputs. As I noted above, Obama has not yet behaved like a Democrat; his policies on the bail-out have been essentially the same as those of his predecessor who most certainly was no Democrat.
So in the end, here's what I say to Megan McArdle: ask your question again when Obama starts doing things a Democrat would do. Pushing through the "public option" on a healthcare would be a start.
So Obama doesn't count because he's not really a Democrat. But Bill Clinton was. But Richard Nixon--the chap who implemented price controls and massively expanded Social Security and Medicare--was definitely a Republican. Jimmy Carter, who deregulated like mad: definitely a Democrat.
What are these policies that neatly define Democrats to exclude only the ones who happen to have crappy growth? On what metric does Barack Obama register as farther to the right than Bill Clinton? Because from what I remember of the 1990s, I spent most of the decade listening to my genuinely left-wing friends weep that he'd betrayed them. Remember Edelman's resigning in protest of welfare reform?
I submit that there is no intellectually credible way that you can throw Obama out of the sample because he didn't include a public option in the health care plan--something that's actually driven by Congress, anyway--while keeping Clinton, his welfare-reformin' ways, and his lovely, lovely late 1990s growth rate. On actual domestic policy, Bill Clinton was well to the right of Richard Nixon. You could argue that he was more socially liberal, but then Jack Kennedy was less socially progressive than Richard Nixon--time is a more important variable than party identification. And at any rate, I'm aware of no work indicating that abortion policy is an important factor in economic growth.
Now, of course, it may be that the economy starts growing like gangbusters in the next year. In which case I expect that Cactus and his merry band of madmen will continue with their arguments. But if, as most people expect, growth continues to stall for the next few years, it seems I can look forward to more explanations of why Democrats--and only Democrats--can be thrown out of the sample if they have low growth and betray The Faith; and why the economic results of Democratic presidential administrations--and only Democratic presidential administrations--are sensitive to exogenous starting conditions.