If Kerry wins, some conservatives and the more principled leftists (yes, there are some) will continue to oppose the War on Terror, and the less principled conservatives will snap out of their four-year daze and see the insanity of what's going on.
Kerry will run again in 2008 against a Republican, and people will increasingly see how little it matters which party is in charge.
But if Bush wins this November, I would bet even money that 2008 will see a campaign between someone in Bush's cabal and Hillary Clinton. The next four years will likely be ugly, whether Bush or Kerry is president. Hillary would win in a landslide against a Republican running on Bush's miserable eight-year legacy.