Politics is about moments. Senators Barack Obama and John McCain are all the proof you need. McCain has been trying to re-create the magic of 2000, despite the fact that he lost that year. When he tells audiences to get ready for "straight talk," he sounds like a rerun of a canceled TV show.

In politics, there's no such thing as a second first date.

Meanwhile, Obama is having a fantastic first date with the American electorate, as evidenced by fundraising numbers that are as spectacular as McCain's are abysmal.

People usually don't want to date someone they've known for a long time. It's common to be romantically interested in the new guy or gal in the office; it's more rare to suddenly think Bob from accounting is a dreamboat after working with him for 20 years.

This is one of the great tensions between the dynamics of the primaries and the general elections. Winning the nomination usually requires building up a network of support among the rank and file. People who've been around a long time are usually best equipped to do this. But people who've been around a long time are usually the least appealing candidates when it comes time to run in the general elections.

In 2003, The National Journal's Jonathan Rauch floated what he called "the Law of 14." "With only one exception since the presidency of Theodore Roosevelt," Rauch wrote, "no one has been elected president who took more than 14 years to climb from his first major elective office to election as either president or vice president."

Lyndon Johnson was the one exception, taking 23 years to go from his first election to the House to the vice presidency in 1960. Of course, that squeaker of an election was an outlier in that all sorts of longstanding trends were in peril. For example, had Nixon beat Kennedy, he would have been the first sitting V.P. elected straight to the presidency since Martin Van Buren.

Indeed, vice presidents and legislators (particularly senators) have similar problems in that they don't seem like men of action. Veeps are usually yes-men while senators are "yes, but" men. They talk and talk, defining their leadership in terms of co-sponsoring this and seeking cloture on that. Worse, most senators who run for president do so after hanging around for a long time. Not only does this mean their shelf life tends to expire, but they only become more senatorial.

Here's a tip: You will never hear the words "President Christopher Dodd."

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.