Question for political scientists

Political scientists who descend to the blogosphere are at pains to make the rest of us understand that, to paraphrase Yogi Berra, 95 percent of electoral combat is half structural -- that is, national election results are driven mainly by the state of the economy, at least in peacetime.  Candidates' skills and political strategy matter only at the margins -- though in a close election, the margin can be decisive. 

This structural view escapes determinism only to the extent that a) an incumbent can, in fact, affect economic conditions, including via ultimately destructive short-term jolts such as Nixon's imposition of wage and price controls; or b) economic conditions are mixed enough, or other factors such as war are salient enough, to put an election up for grabs (as in, for example, the recession we didn't know we had entered in late 2000, or Americans' unease with the course of the Iraq War in 2004).

Of course conditions are often mixed and murky. Nonetheless, I'd like to test the determinism of political scientists struggling to educate journalists and the rest of us, such as Brendan Nyhan, Jonathan Bernstein, John Sides & friends -- and of those who taken their data to heart, such as Jonathan Chait, Ezra Klein, and Matthew Yglesias. Oh, and one who defies category, master of probability Nate Silver.

So here's the challenge: using an economic measure of your choice, such as growth or shrinkage of personal income, or GDP, or unemployment (often dismissed as a lagging indicator), is there a threshold below which you would be prepared to say that Obama cannot be reelected?  Hedge it how you will: exclude military or environmental emergency or disaster, or terrorist attack,  or Republican nomination of a nutcase...hell, make it "Obama can't beat Romney if..." if you like.  And let's not make this too easy, as in Depression-level GDP shrinkage or unemployment.  Hewing as close to our current bad-normal conditions as possible, what's the can't-win economic marker for Obama?

UPDATE 11/3: Nate Silver created for himself a more sophisticated version of this challenge, gaming out various 2012 scenarios while averaging out incumbent's approval rating, GDP growth (or lack thereof), and challenger's ideological rating (moderate to extreme). Given Obama's current approval rating, with a candidate in the historical middle of the ideology scale -- Romney -- Silver rates the challenger's odds at 83% if there's 0% GDP growth over the next year, and at 40% if there's 40% growth.  That indicates almost a tossup given perhaps the likeliest scenario: 2-3% growth.  I must say, I find Silver's 3-factor model for prediction intuitively satisfying, finely calibrated -- check it out.