I'm not so sure about that. He might have a point if he took that approach, but the Republicans are almost certainly guaranteed to run a disgustingly nasty campaign against Obama, whichever one of them wins. Playing the blame game right back could just make it look like a choice between two assholes. Trying to stay positive might make Obama look better by comparison.
The thing is, what would be nice to work and what does work in politics don't usually match.
Obama benefited greatly by what was effectively an incredibly negative campaign against Bush in 2008 - that Obama had nothing to do with it. It was the economy.
The fact the country so clearly blamed Bush for blowing up the economy gave Obama the benefit of a 'negative campaign'. He wasn't doing as well even against McCain/Palin, as terrible a ticket as they were, as bad as Bush had been, before the economy crashed. Negative campaigining is what works.
Let me try a math explanation.
Obama's approval rating is 26%. Congress' is something like 17%, as are, let's say, Republicans.
Now, is Obama going to do better appealing to the 26%? Or to the 75% opposed to Republicans?
This is where people don't vote 'for', they vote 'against'. Run for and you lose, as Obama did so terribly in 2010.
We can easily have a repeat of 2010 - the Republican are playing that game, keep the country down and get the people to vote against Obama for them.
If the country isn't against the Republicans for that, how can Obama win? One thing in Obama's favor is the Republicans nominating only monsters so bad he has a chance.
But even some of these monsters have an ability to win over too many Americans - the 'smooth' Romney, the huckster Perry, that big buck marketing can polish. See Bush.
That draft-dodging power-abusing insider-trading coke-distributing creep was turned into some 'hero of America' to many, many Americans.
Karl Rove saw his pretty blue eyes and picked him to market. Rove also at one time picked Rick Perry and created him as a larger figure. (Thanks, Karl, from the US).