Frank, I think you're off base here, even if you're right about world opinion changing regarding Obama (as is domestic opinion). Because the American president has essentially presided over much of the entire world and the various political disputes and challenges among its many allies and foreign constituencies since WWII, you could make a similar list of anyone who has occupied that office -- all of the things that they have failed to do at a certain point in time without comparing it to accomplishments.  Only 2 years at the job, I think it would be hard to find a president since FDR (inclusive) that has actually accomplished as much internationally, as well as domestically, as Obama has, faults notwithstanding.  If not Obama, then who? What past American president has actually done more in a comparable amount of time?

But furthermore, this rationale that you provide is just not supported by any honest appraisal of world affairs today:

And there is one other vital difference.  Whereas before the US was the unrivalled world military, economic and political superpower which could attempt to do more or less what it wanted, now the US is being challenged both economically and politically.  What you could get away with before is often no longer possible.

Really? What world are you talking about? In what spectrum is America less dominant today than before the fall of the Soviet Union, for example? All that seems to be the case here is that, because of Bush's mistake of invading the wrong country, social discourse is now a bit more critical of America than it was at its low-water mark at the end of the Clinton administration. But that just means that it's more like it was in the 1980's than the 1990's.  Big deal. Especially since Obama opposed the invasion and is, so far, successfully extricating American forces from that country by his own timeline.  That's real power at work, so I just don't see the the evidence to back your thesis, and the street chatter, that Obama is an ineffective leader.  So, who, if not Obama, would you have for your emperor, given that the world apparently still feels it needs emperors?