From Andrew Gelman:
Red State, Blue State, Rich State, Poor State » The nonpuzzle of the close election polls: [W]hy is Obama up only slightly in the polls? This is the question that is (currently) puzzling the political world. A quick answer: the economy isn’t going so badly by historical standards, and Bush’s popularity isn’t so relevant given that he’s not running for reelection. Forecasting models based on past elections predict Obama to get something like 53% of the two-party vote–but these forecasts aren’t perfect; they have a margin of error of a few percentage points. (This is not the margin of error of the polls, arising from sampling variation; rather, it’s the uncertainty reflected by the imperfections of the forecasting model as applied retrospectively to past elections.) In short: macro conditions for the Republicans are not so bad as all that: Obama is a legitimate favorite but there’s no reason to expect that there would be a landslide. Things are going about how one might suspect based on historical patterns of the economy, incumbency, and presidential elections.
To think about it another way, consider this graph adapted from Doug Hibbs... the incumbent party sometimes loses but they never have gotten really slaughtered. In periods of low economic growth, the incumbent party can lose, but a 53-47 margin would be typical; you wouldn’t expect the challenger to get much more than that. Such things can happen (see, for example, Eisenhower’s performance against Stevenson in 1952) but it wouldn’t be expected.