Jon Chait thinks Giuliani and Lieberman would make a formidable ticket in 2008. Jon writes:
Lieberman strikes me as both a shrewd choice for Veep and a likely candidate to get it. Republicans have been largely discredited, so their best chance is to persuade the public that the Democrats are not a legitimate alternative. Lieberman, as a nominal Democrat, would have the best chance of making the case that his party has been taken over by a left-wing fringe. Indeed, that's pretty much what he's saying now.
I'm skeptical. Lieberman would have been a good choice for VP a few years ago, when there was a sense in the country that maybe Democrats had gone too far left on foreign policy. But with the war going so badly, what exactly would Lieberman say? I think the Republicans will have a much better chance next year if they attack the Democrats for being soft on cultural issues, rather than for being weak on terrorism.
Sure, five years from now Democrats will likely be vulnerable once again on national security, but for the time being the GOP does not need a Veep known primarily for his support of the war.