I don't think that anything other than an election will 'take down' Obama. Not when the media is a de facto arm of the Government.
True enough. My concern is the same one that I had in 2008. The public was all frothed up in their disapproval of Bush, and they are heading that way with Obama. This kind of emotional reaction creates an overcorrection the other way, and an "anyone but them" mentality. More edge case candidates on the other side get mindshare and airtime. Or in Obama's case, the public might jump at a candidate they know little about.
In any event, a lot depends on how quickly the money donors narrow the field, as early money is basically burned in the process. The R's best shot is probably Romney, and the D's, maybe Hillary, if (and its a big if) the internal polling shows enough uncertainty about Obama that the "anyone but them" effect would actually benefit a candidate in the same party. It would be an interesting risk.