Well, in a hypothetical match-up anyway. Politico has the PPP story [correction: I meant only that Bush beat Obama in the sense of the growing, overall perceptions - not in actual numbers]:
"Perhaps the greatest measure of Obama's declining support is that just 50% of voters now say they prefer having him as President to George W. Bush, with 44% saying they'd rather have his predecessor. Given the horrendous approval ratings Bush showed during his final term that's somewhat of a surprise and an indication that voters are increasingly placing the blame on Obama for the country's difficulties instead of giving him space because of the tough situation he inherited. The closeness in the Obama/Bush numbers also has implications for the 2010 elections. Using the Bush card may not be particularly effective for Democrats anymore, which is good news generally for Republicans and especially ones like Rob Portman who are running for office and have close ties to the former President."
The Democrats had better tread lightly from here on in. The "It's Bush's Fault" gambit won't work anymore.