On net President Obama has a weaker grip on his 2008 voters than the Republican party has on theirs. According to Gallup, 9 percent of people who voted for Obama in 2008 now back Mitt Romney. In comparison only 5 percent of people who voted for John McCain now plan to support Obama. From Gallup:

Vote Preferences in 2012 Election Based on Reported Vote in 2008 Election

The result is not particularly surprising. Even many of Obama’s strongest defenders will admit that he did not live up to the hype and promise he offered in 2008. Several promises remain unfulfilled, the economy remains weak, and instead of a post partisan presidency we have had perhaps the most partisan four years in Washington.

Given that Obama won by a large margin 2008, he can afford to lose votes compared to the last election, just as long as he doesn’t lose too many.

What I think is interesting about the poll is that it shows just how narrow the number of true swing voters really is. Despite some very big changes in policy, the economy, politics and the official positions of the parties, only a small sliver of voters has moved from one party to the other.