I don't think it's illegitimate when weighing candidates to think of how potential general election pair-ups could change the dynamic of the race and even the meaning of various candidacies. A Giuliani-Clinton race would, to my mind, be the worst dynamic. It would put the red-blue struggle on steroids, and Giuliani's relative secularism would lead him to wage an even more bellicose and authoritarian campaign on the terror issue to win over the Christianist right. He'd be out Jack Bauering Jack Bauer - an unnerving prospect.
Clinton-Romney would be a less poisonous battle, but arguably more divisive. Romney's Mormonism makes it more likely that he will go overboard on the social issues to appease the evangelicals, and in a Romney-Clinton match-up, you just know that Bill Clinton's entire personal life for the past seven years will be dragged out into public viewing. The GOP will try to engineer it so that the issue becomes a choice between a corrupt, boomer marital "understanding" versus a fecund, high-school sweetheart, gee-whiz marriage. Again it will be culture war central - although I'm not sure who would win. Still, if you want to relive the Lewinsky years in wartime, you know who to vote for. On Iraq, Romney is so protean he will be able both to potrray himself as Bush's successor to his base, while signaling he will withdraw from Iraq and restore government competence in Washington to the middle. McCain-Clinton would bring out the worst in McCain, but would also, I'd say, lead to an easy McCain victory. Epecially if, as I'd bet, McCain makes Huck his veep to bring home the base. Clinton vs Huckabee would be a landslide for her, I'd say.
Put Obama in the mix and the general election mood changes drastically.
An Obama-McCain race would, as I wrote yesterday, be the best outcome in terms of actually fostering a civil, constructive debate about the future. It would also instantly create a narrative of young versus old. Obama could counter by making Biden his veep, neutralizing his foreign policy vulnerabilities. McCain could add Huck to tilt younger, and then play the experience card relentlessly. He could also run as a one-term president - we'd even believe him. Obama-Romney would be more polarizing (though no one polarizes like the Clintons), but the ugly anti-Mormon bigotry and racism would probably cancel each other out. Obama vs Huckabee would be fascinating, but I'm afraid I cannot see Huckabee gaining enough credibility on foreign affairs or economics to become president.
If I were a Republican, I'd say McCain is easily the strongest general election candidate. If I were a Democrat, the potential dynamics of a race between Clinton and a Republican - almost any of them - would make me pick Obama.