"no prominent centrist Democrats have openly defected from Democratic ranks on the basis the 'Obama threat.'"
In the historical precedents I cited, and in connection with any attempt to find a basis for the differential defection rate, it was/will be necessary to inquire of voters if the Goldwater, McGovern, Nixon, Palin or Obama 'threat' was the specific reason for their defection. In the case of the Republican defectors I named, the cause of the defection was specifically cited as Palin. I cannot recall that Lieberman ever indicated that the basis for his defection was Obama, i.e. he would have defected whether the nominee was Obama, Hillary or any other of the Democratic primary contenders. Have any of the other Democratic defectors you have named specifically attributed their defections to the fact that Obama, rather than any other Democratic primary nominee, was chosen for the Presidential contest? For purely PR reasons, it would seem unlikely that they would have done so because to have done so would have put them on the spot with respect to a charge of racism -- however many other reasons (e.g., his liberal-left politics) they might have cited.
In the first instance I cited, it was widely believed that Stevenson's candidacy was seriously weakened by the public's association of Stevenson with public jesting. Stevenson was a genuinely witty individual who knew how to deliver a humorous line regardless of who wrote it. The arguments given in support of the perception that this was a handicap for Stevenson generally focused on the notion that the issues of the day were too grave to allow for public jesting. It will be interesting to observe whether or not this same perspective re-surfaces in connection with Palin's appearance on SNL. Democrats are nothing if not foxy: it was Nixon's ability to finesse them at their own game that led to his occupying a unique place in their hall of infamy.
The PUMA defectors would have supported Hillary.
It is interesting that you claim Stevenson lost support because he was thought of as a clown. I thought his more common perception was as an all too serious policy wonk a la Dukakis.