Pretty much everyone seems to agree that President Obama's biggest electoral handicap is the economy. Pretty much everyone also seems to agree that, actually, assigning blame (or credit) for the economy entirely to a president is unfair for a couple of reasons: (1) the president doesn't have anything like full control over national policy levers; and (2) national policy levers don't have anything like full control over the economy, which is also influenced by things ranging from technological change to international boom and bust cycles.
There's no easy way to tease apart all these factors. But there's one simple operation that partly clarifies the picture, and it's especially useful when the economy suffers from troubles that are international in scope: compare the performance of the American economy to other similar economies--i.e., "mature" market economies, with high per capita GDP. With that in mind, take a look at this graph of unemployment rates over the past 12 years, and at my comments on it below.
Focus on the maroon, yellow, and light blue lines. (You can ignore the dark blue line, because it represents the 17 Eurozone countries, which are included in the 27 European Union countries represented by the yellow line.) Let's look at three phases:
 The Bush years. When George W. Bush took office, US unemployment was a bit lower than Japan's and about four percentage points lower than Europe's. When he left office, US unemployment was about the same as Europe's and about four percentage points higher than Japan's. So: things got way worse compared to both Europe and Japan.
 The Obama years. When Obama took office US unemployment was one fifth of a percentage point below Europe's, and as of last month it was more than two percentage points below Europe's. When Obama took office US unemployment was 3.6 percentage points higher than Japan's and as of last month it was four percentage points higher than Japan's. So: things got much better compared to Europe, somewhat worse compared to Japan.
What do you see as of May? You see the sharp upward slope of the unemployment curve, which Obama inherited from the Bush years, begin to moderate. And within five months the unemployment rate has reached its acme, after which it drops considerably. Of course, it would be great if it had dropped further, but it would be a lot easier to blame its failure to drop further on president Obama had Europe's unemployment rate, in the same period, not risen by a full percentage point; or had Japan's unemployment rate fallen more sharply than, or even as sharply as, America's.
Again: Neither Obama nor Bush is, in fact, anywhere near wholly responsible for the performance of the economy during their administrations. For example, both Democrats and Republicans, as well as other players, bear some blame for the housing bubble whose bursting marred the end of the Bush years and the beginning of the Obama years.
Still, the first step toward getting clearer on what a president is responsible for--especially during a period of global economic trouble--is to compare America's performance to the performance of similar economies. By that standard, Obama's handling of the economy starts looking pretty good. Maybe not so good that it should be a big political asset, but good enough that, in a just world, it wouldn't be much of a liability.