Sarah Palin could well be a game-changer, one way or another.

Even given the truism that few people vote for or against the vice presidential nominee, John McCain's selection of the Alaska governor as his running mate has the potential to be the exception to that rule.

Perhaps more important, the selection of Palin, who is virtually unknown outside the Great White North, will put to rest any notion that McCain is just another Washington politician rooted in the past and unable to think outside the box.

The question is whether his pick of the 44-year-old Palin will - in the end - be seen as a stroke of genius or a pick by an older guy waiting for a stroke.

The next few weeks will be a furious race between the Republicans and Democrats to define Palin for the American people.

Without doubt, no one in modern American history has been so little known before being tapped as a vice presidential candidate.

Moreover, her Alaskan upbringing and lifestyle will be viewed as both a curiosity by some, and probably weirdness by others.

The threshold question is whether by November - not necessarily by tomorrow - voters decide she could be president of the United States if something happened to McCain.

If so, she has the potential to help with key voter groups - women, hunters, rural residents and social conservatives, the last of which have been skeptical of McCain.

If not, McCain's candidacy could well be toast.

Democrats will no doubt seek to raise questions about Palin's fitness to be a heartbeat from the presidency - especially since at 72, McCain would be the oldest person ever elected president for the first time.

But they will have to walk a tightrope in that effort for two reasons:

First of all, doing so could undercut their own presidential nominee, Barack Obama, who is only two years older than she and has only a slightly beefier resume.

And then there is the problem of running against a woman, which Obama discovered during his Democratic primary campaign against Hillary Clinton.

Obama is most likely to be successful in raising questions in the minds of many voters in urban areas, which are reliably Democratic, but her potential would be greatest in rural and exurban America and could be helpful in some important battleground states.

Palin has been governor of Alaska for two years, winning office by beating a scandal-plagued governor of her own party with a campaign keyed to ending government corruption.

Politically, she touches myriad bases both for Republicans and, most important, swing voters, especially, but not exclusively, women. Not necessarily Hillary voters - who may be ideologically uncomfortable with her conservative views - but, more crucially, suburban women who struggle to balance work and family and may instinctively want to give a sister a chance.

Palin, who calls herself a "hockey mom," has five children, and went ahead with the most recent one earlier this year after genetic tests told her the child would have Down syndrome. Her husband is part Eskimo, a commercial fisherman, oilfield worker and union member.

Palin, who hunts, fishes and likes moose burgers, is a marathon runner and conservative Christian whose oldest son is to be deployed to Iraq next month.

She is both a former beauty queen and high school point guard who earned the nickname "Sarah Barracuda" for playing the state championship game, which her team won, with a stress fracture in her ankle.

Unknown is whether the early negative reaction by Washington insiders, including many Republicans, that she undercuts McCain's campaign theme that he has the experience that Obama lacks to guide the country in a dangerous world will become the thinking in the heartland.

The pick surprised many analysts who had expected McCain to take a "safe choice," probably a white male governor or senator well-known to the media because, after trailing by double-digits in national polls, he had closed to a dead heat entering this week's Democratic convention.

But McCain ultimately decided that, given the ailing economy, unpopular war and less popular GOP president, all of which benefit Obama, he had to take a riskier path.

Peter A. Brown, formerly the chief political writer for Scripps Howard, is the assistant director of the Quinnipiac University Polling Institute and a weekly columnist for The Wall Street Journal online.