Via a colleague:
If CBO’s projected real GDP growth for 2012 (3.4% in Table C-1) is correct (and the administration and Fed projections seem to be in the same ballpark), Obama’s expected popular vote margin is 8 percentage points, +/- 7 (i.e., 85-90% chance of reelection). If GDP growth in 2012 turned out to be half of that, Obama would still be a 2-to-1 favorite.
The economy is not “all that matters” by any means. For one thing, there’s that +/- 7. Also, my projection takes account of how long the incumbent party has held the White House. Obama’s ace in the hole is that it is very rare for a party to lose after just one term. Given that fact, Obama doesn’t need a great deal of economic growth (or anything else) to be favored for reelection.