The Barack Obama who stood on the debate stage in Denver Wednesday night was virtually unrecognizable to the person who swept to victory in 2008 or even the man who had built a narrow-but-clear edge in the 2012 race.

In the wake of the debate, Democrats conceded that former Massachusetts governor Mitt Romney had dominated the proceedings and quickly moved on to a deeper -- and harder  to answer -- question: What the hell was wrong with the president?

Theories abound -- up to an including that Obama, who had done debate prep in Las Vegas, struggled to adjust to Denver's altitude. (Not kidding. That is a theory espoused by none other than former Vice President Al Gore.)

Below we take on a few of the likeliest reasons for Obama's out-of-character performance -- culled from conversations with Democratic strategists and our own observations.

* He's not used to being challenged: For all of the chatter that President Obama loves to surround himself with people willing to disagree with him, the truth of the matter is that being the leader of the free world means that what you say goes pretty much all the time. And, being challenged by staff -- no matter how senior those staff might be -- is not the same thing as being directly confronted by someone at your level and who wants your job.  Obama seemed to vacillate between flummoxed and annoyed that Romney was contradicting almost every assertion he tried to make. Sidebar: That "Well, I am the president" tendency goes a long way to explaining why incumbents -- George W. Bush, Ronald Reagan, Jimmy Carter -- tend to be regarded as losers in the first general election debate.

*  He was tired: Obama looked world-weary at best and downright grim at worst during the debate. And, his inability (or unwillingness) to go after Romney's record -- or rebut some of Romney's claims about his own -- gave off the impression that Obama just wasn't well prepared enough to do so.  It's understandable for a president to be a little worn down -- he is the president after all -- but it's up to he and his staff to understand that these debates matter a lot and that just assuming you will be able to fake it until you make it isn't good enough.

* He's not that good a debater: Over the weekend, Obama told a crowd in Nevada that he was only an "okay" debater. At the time, the political world rolled its eyes -- chalking it up the sort of expectation lowering that is now de rigeur in these sorts of political moments. Turns out, Obama might have been giving an honest assessment of his debate abilities.  As last night's debate was unfolding, we thought back to the 2008 general election debates where Obama was solid but far from spectacular.  Or even to the 2008 primary debates where, for the most part, then Sen. Hillary Clinton outperformed him.  Obama is, without question, the most gifted speaker in politics right now. (Bill Clinton is not, technically, in politics at the moment.)  But, as a debater, he is not in that same elite class.

* He got screwed by Jim Lehrer:  On the morning after the debate, the most prominent talking point on the left was that moderator Jim Lehrer had aided and abetted Romney's victory by refusing to enforce the rules of the debate or inject facts when the Republican was clearly straying from them. While the debate was clearly not Lehrer's best moment, it's hard to lay all the blame for Obama's poor performance at the moderator's feet. If Obama felt like Lehrer wasn't doing enough to fact-check Romney in real time, why didn't he take it upon himself to do it?

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