Here's something to think about. Given the likelihood that social conservatives will reject John McCain as the Republican standard-bearer in 2008, what are the chances of an "independent" McCain-Lieberman ticket in 2008? Mark Schmitt has a brief thought about it. I think it would be hard for anyone who's not a billionaire to mount a nationwide third-party challenge in the next election, so I'm skeptical about the "revenge of the independents" argument.
Nonetheless: I think a McCain - Lieberman ticket would spell doom for Republicans in 2008, mostly because it would split the committed Republican vote in most battleground states. No liberal in his right mind would vote for McCain and Lieberman in a Presidential election. A liberal platform advocating fair trade, a balanced federal budget, preserving Social Security in its present form, disengaging from Iraq, and taking action on global warming would hold the base together. On the other hand, a base conservative platform calling for more tax cuts, privatizing Social Security, building a border fence, banning gay marriage, and maintaining troop levels in Iraq would face some crossover from McCain's likely platform.
Let's say, for the sake of argument, that in 2000 the Democratic nominee is Barack Obama (with Russ Feingold as VP) and the Republican nominee is Mitt Romney or Sam Brownback (both more likely than Giuliani or McCain). With a competing McCain - Lieberman ticket, I suspect the winner would need only about 40 percent of the vote to win any given state. In a three-way race, I can see Obama - Feingold getting to 40 percent in places like Ohio, Florida, Pennsylvania, Iowa, Missouri, Colorado, etc. I think it's tougher for Romney - Brownback to get there.
There are definitely not more liberals than conservatives in the U.S., don't get me wrong. But I think there are an equal number of committed Democrats and Republicans in lots of states, and I see more trouble for Republicans than Democrats from a McCain- Lieberman challenge in those states in 2008.
TURKEY DAY UPDATE: I came across more speculation about McCain- Lieberman here, and here. I'll add more links as I find them. Most of the speculation about the pairing comes from Lieberman's decision to hire Marshall Wittmann, the proprietor of the Bull Moose Blog, as his new spokesman.