Obama hasn't been a disaster for his constituents at all. He's been very successful at getting left-wing policy passed. If we had a right-winger who was as effective as Obama at passing his agenda we'd be very happy. So I'm not worried if a candidate was a governor, senator, house member or private citizen. There are far more important factors to look at.
I suspect that Obama has been a disaster to all but his core voters. That is why the midterms were as they were. That is why the Dims lost Weiner’s House seat. That is why Obama may very well not be reelected. That is why I read an article today about the youth vote abandoning him.
You may not worry about the nominee having executive experience, but the electorate at large may well. In a brokered convention the professional pols will be concerned with winning so that will be a factor. Also issue of having run and won in a statewide race will matter even more if the nominee is from a brokered convention and was not picked by winning a number of states in primary elections. That makes any Congressman or ex-Congressman or mayor or ex-mayor unlikely.