Noam Scheiber says "I talked to eight or so GOP operatives for my Pawlenty piece and brought up Lieberman's name almost every time. About half thought it was a lunatic idea, the other half thought it was a decent idea but still highly unlikely."
Probably everyone's right. As with the previously discussed notion of a McCain-Huckabee ticket, a McCain-Lieberman ticket would be courting disaster -- you would confirm a lot of the conservative movement's fears about McCain, you'd be doubling-down on McCain's support for an unpopular war, Lieberman's not a charismatic guy or effective campaigner, etc.
But I continue to think that if McCain makes his top priority avoiding disaster, then he's just dooming himself to defeat. There's something plausible to be said along the lines of "most people think Bush is a bad president and I agree with them, but Democrats have responded to GOP unpopularity by positioning themselves substantially to the left of where they were in the late 1990s and nothing about Bush's record actually justifies that kind of move." Joe Lieberman is pretty well situated to try and make that argument. But to make it persuasively, McCain would really need to move to the center on non-war topics and make some decisive breaks with the Bush administration. Thus far, he seems to be doing the reverse -- sapping his dissent from Bush on climate and executive power of any content -- which I do think is the safe strategy, but timidity plus extremely unfavorable objective conditions are a recipe for losing.