Tim Oliphant explains how unprecedented a Kerry/McCain ticket would be and outlines some of the difficulties:
Clearly, even the prospect of McCain under serious consideration for some kind of "national unity" ticket would be an earthquake event; this is something that has not happened in the 200 years of party elections in this country. For it to progress as an idea, discreet big shots would have to labor behind the scenes to construct an alliance; this is not simply a question of whether Kerry should consider asking McCain to serve or whether McCain should consider serving. In a way, that's the easy part, almost superficial.I do think a lot of the talk about a Kerry/McCain ticket seriously underestimates the historical significance and political complexity of such a move. But I'll go even further: I think the kind of coalition Oliphant is talking about is actually impossible. John McCain just doesn't have the political strength to bring any Republicans into the fold on this. Sure, he's trustworthy and shoots from the hip, but those are the same qualities that make him a maverick, an outsider in the Republican party. If he can't bring anyone with him, he just ends up looking like a traitor, which damages his credibility and belies any talk of "national unity".
The hard part would be defining the nature of such an unprecedented alliance. There would have to be a discussion about federal judges and about every major foreign and domestic challenge facing the country. It would be silly for them to bargain over policy details as if they were crafting legislation; however, it would be essential for them to agree on a basic approach and program, or the alliance would have no meaning.