After John McCain picked Sarah Palin as his running mate, some people saw the origins in McCain's love of craps, a game involving little in the way of strategy but a willingness to take big risks. McCain was quite unusual in his penchant for risk-taking; the life of a politician, where your words are watched closely and there is always a whole party of people out to destroy you, not to mention the fact that you constantly have to appeal to ornery voters, inclines one toward caution. As Time's Adam Sorenson says, in today's campaign, "Every semi-public utterance will find its way into the news; every available scrap of personal history will worm its way to daylight. That’s why we end up with candidates like Romney and Obama, men of catalog-perfect families, immaculate pasts and abundant political caution." I'd actually argue that Obama has exhibited what might be called a general cautiousness punctuated by episodes of extreme boldness, none more so than his decision to seize the moment and run for president just a couple of years into his first term in the Senate.
But the modern campaign doesn't produce cautious candidates so much as it constrains the behavior of candidates who would probably be about as successful or unsuccessful in different circumstances. Mitt Romney didn't beat his primary opponents because he was the most cautious candidate; it wasn't just the occasional gaffe hurtling around Twitter or an intemperate remark caught on someone's cell phone that did the other candidates in. I'm pretty sure that even in a campaign waged by means of semaphore flags we would have discovered pretty quickly that Rick Perry was an idiot, for example, or that Michele Bachmann was a nutbar, or that Newt Gingrich was, well, Newt Gingrich.
That being said, there are certainly going to be more incidents during the campaign in which Romney or Obama (or both, most likely) get engulfed in some phony controversy over something that nobody would ever have heard about a couple of campaigns ago. That has to make them look over their shoulders a lot, and you have to sympathize. When you're running for office, there are just so many different ways to get screwed. Even if you're incredibly careful, one of your aides can say something foolish, or hell, somebody who doesn't even work for you can say something foolish, and you're the one who has to pay the price. The higher the office you're seeking, the more people there are around you who can screw up, and the more people on your opponent's side and in the media who are waiting for you to screw up. And the stakes at the presidential level are so high—by the end of it, either you'll be the most powerful person in the world, a figure about whom people will be writing books for decades, or you'll be the butt of a thousand jokes and reviled among people who were once your supporters and friends. It'd sure make me cautious.