Marco Rubio is a young man in a hurry. A mere 43 years old, he faces a choice next year. He can run for re-election to the Senate, a race he would probably win, then keep building a legislative record in preparation for one of the seven or so future presidential elections he'd be able to run in before he gets too old. Or he can give up his seat and take his presidential shot now, even against some better-funded candidates, knowing that if he loses, it could become his only chance to reach for the brass ring. He may have made up his mind; according to an article in Tuesday's New York Times, Rubio "is quietly telling donors that he is committed to running for president, not re-election to the Senate." Fortune, Rubio apparently hopes, will favor the bold.

If you're wondering why Rubio might think he has a shot, look no further than the current occupant of the White House. When Barack Obama started seriously considering running for president in 2008, the conventional wisdom held that the notion was ridiculous. Someone who had been in Washington less than four years, with only a couple of bills to his name? How could he be so presumptuous?

But Obama — whose career has been marked by long periods of caution punctuated by audacious risk-taking — recognized his own talents, and the fact that the moment was perfect for a candidate like him. And for all that Republicans may think he was unprepared and inexperienced, they've assimilated the idea that there's nothing wrong with a candidate like Obama seeking the nation's highest office. This year there are three Republican senators in their first terms who will be running — Rubio, Rand Paul, and Ted Cruz — and though they each have admirers and detractors in the Republican Party, nobody's saying they haven't put in enough time in subcommittee hearings.

There are still plenty of older senators around — the median age of the current Senate is 61, and there are 23 in their 70s or 80s — but these days when we think of a presidential candidate emerging from Congress' upper chamber, we no longer imagine someone who has spent decades amassing a legislative record, like so many party nominees before, in the mold of John Kerry or Bob Dole. Instead, the senators who today decide to make a run for the presidency are the ones who just got there and can't seem to wait to get out.

That may mean we've stopped thinking that a deep understanding of legislation is much of a requirement for the presidency. Once again, Barack Obama may have proven this point. Despite his modest résumé as a lawmaker, his first two years in the White House saw the passage of an extraordinary number of significant bills, none more so than health care reform, a decades-old Democratic priority that Bill Clinton (with his famous people skills) failed to achieve. Though the model of the legislator-as-president was Lyndon Johnson, people forget that the Great Society was made possible by the fact that Johnson enjoyed huge Democratic majorities after 1964, with over two-thirds of the seats in both houses held by his party. Passing all those programs still wasn't easy, but it would have been impossible in a more closely divided Congress, let alone one controlled by Republicans, no matter how good a persuader and vote-counter Johnson was.

Governors still get the benefit of the doubt as potential presidential candidates, on the assumption that leading a state is a lot like leading the country, so they will have accumulated the skills and experience to do the job well. But if we no longer think that time in Washington — learning about national and international issues, being around in times of crisis, coming to know how the government works and how it can be mobilized to one's ends — is all that important, what are we looking for in a president? Is it a job that requires a kind of abstract skill set, which we might not be able to define, but we know it when we see it?

Oliver Wendell Holmes supposedly said of Franklin Roosevelt that he had a second-class intellect but a first-class temperament; many of today's conservatives might say the same of Ronald Reagan. But you can define "temperament" almost any way you want, and see in any character trait the potential for greatness or disaster.

For example, just after 9/11, commentators fell all over themselves to praise George W. Bush's "moral clarity," but before long his insistence on seeing every problem in Manichean terms and his unwillingness to consider nuance seemed like positively catastrophic flaws. It wasn't that we couldn't have figured out in 2000 that this oversimplified outlook on the world was part of who Bush was; at that time we just had no way of knowing how much it would shape his presidency. Today, liberals look at Barack Obama's careful weighing of options and unwillingness to go off half-cocked as a strength, keeping him calm while those around him are saying we should all freak out (remember when there was going to be an Ebola epidemic in the United States?). Conservatives see the same trait as fatal indecisiveness.

Likewise, those in both parties could point to candidates who were amply experienced, but who would have (or did) make bad presidents. You know who had lots of experience in government before becoming president? Richard Nixon. Which, of course, tells us nothing about what sort of president someone like Marco Rubio would be. But it's a reminder of how hard it is to predict what's really going to matter to the next presidency.