Joe Klein of Time explains

Over the past few weeks, Barack Obama has been criticized for the following: He didn't go to Berlin for the 20th anniversary of the Wall's coming down. He didn't make a forceful enough statement on the 30th anniversary of the U.S. diplomats' being taken hostage in Iran. He didn't show sufficient mournfulness, at first, when the Fort Hood shootings took place, and he was namby-pamby about the possibility that the shootings were an act of jihad. He has spent too little time focusing on unemployment. He bowed too deeply before the Japanese Emperor. He allowed the Chinese to block the broadcast of his Shanghai town-hall meeting. He allowed the Chinese President to bar questions at their joint press conference (a moment memorably satirized by Saturday Night Live). He didn't come back with any diplomatic victories from Asia. He allowed Khalid Sheikh Mohammed and the other 9/11 plotters to be tried in the U.S. criminal-justice system rather than by the military. He has dithered too long on Afghanistan. He has devoted too much attention to — and given congressional Democrats too much control over — health care reform, an issue that is peripheral to a majority of Americans.

And all this has led to a dangerous slippage in the polls, it is said, a sense that his presidential authority is ebbing.