As Duncan says, people are losing confidence in Obama's competence.
Electoral competence is what led me to support him in 2008. Starting with Super Tuesday, Hillary increasingly looked to me like she was going to lose to McCain, and Obama looked like he was going to win. (Before that, I thought Hillary was a lock for the nomination.) There wasn't a whole lot of daylight between them policy-wise, and neither were especially liberal. I was tired of losing, so I went with the candidate who I thought would win.
I also thought that it didn't matter whether we had the first black President or the first woman President, what was more important to the world was having a competent President. I hoped Obama would be as adroit at governing as he was at campaigning. That's not working out so well. But where we'd be if we had gotten Hillary instead (assuming Hillary could have beaten McCain) is, as he says, unknowable.
But I do know I'm glad John McCain is not President. And that Sarah Palin is not Vice President. It's knowable that things would be worse if they were in the White House.
A similar choice is going to come up in 2012. I think primarying Obama would be a disaster. But it still might happen. Whether he wins the primary or not, in November there will be a Democrat and a Republican. And the Republican will be, relatively speaking, far worse for the country.
So I expect whoever the Dems put up, I am going to support and work for. I wish it can be with some enthusiasm and optimism, as in 2008. But I am going to do it regardless, because it's the only responsible thing to do.