Parapundit's Randall Parker has been speculating on potential Presidential match-ups and outcomes.
His basic argument is that a Hillary defeat of McCain would be a good thing. Republicans would oppose an immigration amnesty and the further opening up of the southern border to illegal Hispanic immigration, since it would be favored by the Democratic White House and both Democratic houses of Congress (although the Dem-controlled House, especially with regards to its newest additions, is not an entirely anti-nationalist body by any stretch). This would allow the GOP to fight for its demographic life by taking up a popular cause, rather than having to choose between either toeing the McCain Whitehouse line in favor of electoral self-immolation or internecine fighting within the Republican ranks.
I'm mixed on a best-case scenario. The major hangup I have with a McCain defeat is what it will mean for the Supreme Court. Justice Stevens will turn 90 during the next President's term. There has only been one other nonagenarian judge in the Court's history, and he (Oliver Wendell Holmes) left within the year of becoming one. Scalia, Ginsburg, and Kennedy are all in their seventies. With Democratic control of both the Senate and the Whitehouse, that could spell a drastic leftward shift in the court, especially if Scalia retires.
But the worst-case scenario doesn't seem as cloudy. Obama versus McCain, with McCain winning, strikes me as the least desirable outcome of all. The GOP's performance among Hispanics would be maximized, possibly eclipsing the 50% mark, as Hispanics otherwise tending to support the Democratic candidate would be unenthusiastic about voting for a black. Many would stay home, and others would 'defect' to McCain and his pal Juan Hernandez. If you think the WSJ bilge over the essentiality of the Hispanic vote as garnered through support for open borders is nauseating now, just wait.
Further, as Obama is less culpable in the Iraq situation (and would certainly attack McCain on the issue of Iraq during the general election campaign) than Hillary is, we'd also get neocon braying about how an interventionist policy that not only maintains but accentuates our troop presence in 130 countries across the globe is an electoral winner and something the US must embrace. And the US would have to suffer embracing as much for at least another four years.