From Greg Mankiw:
The reason I say "curious" is that there's very little evidence that presidents (or the government more generally) have much impact on the current state of the economy. They can influence long-term performance, but in the short term, the US economy, and others, go through periodic ups and downs that we call business cycles. That was true 200 years ago and remains true today. There's some debate about where business cycles come from, but the president's actions are rarely on the list. Nevertheless, it's become good strategy to claim credit for good news and blame the other guys for bad news -- even though neither is likely.