I don't think the Democratic vote would matter if it were McCain/Lieberman.
Republicans will vote for a Republican President, there's no way the Republicans will have another Ross Perot for a while and the Democrats won't have another Ralph Nader, they'll try and ensure that by telling and scaring their voters that if they vote third party the opposing side will end up in the White House.
The independent/moderate vote would flock to a McCain (Republican)/Lieberman (independent Democrat) ticket. Due to their history of being willing to work with the opposition unlike what George Bush, Nancy Pelosi, and Harry Reid have done. The fact that the ticket is composed of a moderate conservative and a moderate liberal who can work very well together.
And yes Lieberman is a rather moderate liberal, he's pro-choice, pro-gay rights, pro-gun control, supports enviromental efforts, against Social Security privatization, supports a larger effort into health care, etc. His hawkish foreign policy beliefs, his criticism on the entertainment industry, and his willingness to work with Republicans is what gives him the reputation of a DINO.