Historically, the most successful candidates have been governors and generals. In 2004, American Enterprise Institute president Christopher DeMuth crunched the numbers and found that 55 percent of our elected presidents were either governors or generals, while "only three of our 31 elected presidents have come from a primarily legislative background, and none was re-elected."
So you have a nomination process that's biased in favor of people who've spent their careers collecting chits from the power brokers, and a general-election environment that generally wants a fresh face. In 1996, everyone thought former Tennessee Gov. Lamar Alexander was the GOP's best hope for beating Bill Clinton. Instead, the GOP decided it was Bob Dole's turn. That turned out swell.
What does all this mean for today? Well, obviously, it's bad news for McCain. But Mitt Romney, a former governor whose face is so fresh-looking it's like he preserves it in Mylar, looks to be in pretty good shape. Rudy Giuliani and Fred Thompson are both running out the clock of their respective shelf lives, but one could say they're fresher to the American public than that suggests. Rudy was born again on 9/11, and Thompson left the Senate before going stale and has been rejuvenated by the magic of Hollywood.
In other words, if you put the disastrous behavior of the GOP these last few years aside (a big if, of course), the Republican field looks pretty good. Bush may not be the albatross Democrats hope. None of these guys is running as the Bush heir apparent, and no Democrat has won the White House without facing an unpopular GOP incumbent in more than 40 years.
The more interesting question is what this means for the Democrats, specifically Hillary Clinton. If her tenure as first lady counts - and I think for many people, including Hillary, it does - then her shelf life is almost up, too. Indeed, I think the Clinton name may be more perishable than the expiration date suggests.
Meanwhile, the young, fresh-faced, un-senatorial Obama is clearly having his moment. The problem, as McCain can tell him, is that having your moment in the primaries is no guarantee of getting the nomination.