Adam Green makes the point that,
The reality is that the [political] "center" is not an ideological place but a political one, defined not by the nature of a specific policy but the political positioning of the right and left poles of debate. The reason why Democrats didn't produce a more liberal [healthcare reform] bill is because the coalition of preening centrists needed to move right every five minutes in order to locate themselves in the "middle" between a GOP sprinting to the right and a left willing to take major steps in the same direction.
The other problem comes from our use of a left-right continuum with swing voters positioned in the "center". While there are true moderates that are center-left or center-right on issues, the thing that turns close elections is swing voters that I would argue are neither right, left or center, they are largely volatile, late-decider, low-information voters not reflecting a strong political position of any sort: not some notion of center but emotionally reactive to circumstance. And with almost a revulsion of political parties: they hate political parties because of their strong beliefs and tribal cohesiveness, not necessarily because of what they want to accomplish.
For example, an election day poll showed that,
The economy was by far the dominant issue. Democrats were rebuked for the failure to create jobs, but there is little sign that voters embraced the conservative agenda or ideology.
That is, reacting to economic woes, they weren't seeking some ideological difference, but sending a message of frustration which can only be done by blaming those in power at the time.