It's still 11 months until the presidential election, and we haven't had a primary or caucus yet, but I'm willing to make a prediction: the Republican nominee will win.
I have two reasons. First, all the likely Democrat nominees are senators, and the likely Republican nominees are either governors or mayors. Historically, executive branch candidates do much better than those from the legislative branch. To quote Wikipedia's U.S. Presidential Elections page:
Contemporary electoral success has favored state governors. Of the last five presidents (Jimmy Carter, Ronald Reagan, George H. W. Bush, Bill Clinton, George W. Bush), four have been governors of a state (all except for George H. W. Bush). Geographically, these presidents were all from either very large states (California, Texas) or from a state south of the Mason-Dixon Line and east of Texas (Georgia, Arkansas). The last sitting U.S. Senator to be elected president was John F. Kennedy of Massachusetts in 1960. The only other sitting senator to be elected was Warren G. Harding in 1920, whereas major-party candidate Senators Andrew Jackson (1824), Lewis Cass (1848), Stephen Douglas (1860), Barry Goldwater (1964), George McGovern (1972), Walter Mondale (1984), Bob Dole (1996), and John Kerry (2004) all lost their elections.
Maybe this is because of the compromise nature of the legislative process, and the hay that can be made of it. Remember "He voted for the war before he voted against the war"? Maybe it's that personalities that succeed in Congress are not well-suited to executive branch work. I don't know.
The second reason is that both parties have moved to the left. Clinton and Obama are both more liberal than Kerry was, and neither Romney or Giuliani is as conservative as Bush was. That means that the Republicans are moving toward the center at the same time that the Democrats are moving away from it. I've heard more than one liberal Massachusetts friend say that Giuliani wouldn't be bad, for example.
I hope I am wrong, but from this (admittedly early) vantage point, it doesn't look good for Democrats.