Unlike some in the blogosphere, I actually do think Obama's lead is slight enough that there's reason for concern about his performance, or at least reason to ask questions about why he's not pulling away. But unlike some in the media, I think those questions have an answer. Political scientist Andrew Gelman provides it:
This graph actually does show that there are time when candidates matter. In 1952, Eisenhower's personal popularity overwhelms economic growth that should have kept the Democrats in power. But in general, most elections cluster around fairly small margins, and Obama-McCain is currently demonstrating pretty normal numbers.
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.In other words, the chatter about Obama's weak performance actually says a lot more about the way we understand elections to work -- i.e, contests between campaigns, rather than outcomes of broader sociopolitical forces -- than it does about Obama's electoral performance. This graph, also via Gelman, shows some of the historical precedents (click on it for a larger version):
This graph actually does show that there are time when candidates matter. In 1952, Eisenhower's personal popularity overwhelms economic growth that should have kept the Democrats in power. But in general, most elections cluster around fairly small margins, and Obama-McCain is currently demonstrating pretty normal numbers. 
