Though John McCain clearly concluded that Palin could attract female voters and grab his campaign some Barack Obama-style media buzz, he also is taking a risk that in elevating a largely unknown figure, he undermines the central theme of his candidacy that he puts "country first," above political calculations.
For a candidate known to possess a quick temper and an unpredictable political streak, the decision raises questions about how McCain would lead -- whether his decisions would flow from careful deliberations or gut checks in which short-term considerations or feelings outweigh the long view.
"Americans like risk-takers, but they also want to know that in times of crisis, you're going to be calm," said Matthew Dowd, who was a senior campaign strategist for President Bush but is neutral in the McCain-Obama race.
"Americans don't necessarily want somebody in a time of crisis to be overly emotional," Dowd said. "That's the balance that John McCain's going to have to show the public."
The Palin risk also has the potential to reap big rewards.
Her presence on the ticket as a strongly antiabortion mother of five -- her infant son has Down syndrome -- promises to energize evangelical voters who have been skeptical of McCain.
Already, some top conservative Christian leaders who criticized McCain in the past have proclaimed enthusiasm for the pick.
Moreover, as a hunter, a member of the National Rifle Assn. and an avid snowmobiler, Palin appeals to many facets of the GOP base.
McCain's choice of Palin strikes a contrast with Obama's running-mate selection of Joe Biden, a longtime U.S. senator whose foreign policy credentials and working-class roots seemed to fill important gaps in Obama's resume and political style.
That is not to say that voters always want the deliberative approach. McCain's popularity stems partly from his independent style, whereas Obama has been accused of being too professorial and failing to connect with middle-class voters on a personal level.
Still, for McCain, who turned 72 on Friday and has had bouts with the most serious form of skin cancer, the priority in his running-mate selection was picking someone voters could envision becoming commander in chief should something befall him as president.
Or so it had seemed.
As of midweek, according to GOP sources, Republicans believed that the Arizona senator had narrowed his choices to more-seasoned contenders: Sen. Joe Lieberman, an independent from Connecticut, and Minnesota Gov. Tim Pawlenty.
Pawlenty, popular with conservatives, was viewed as the safe bet; Lieberman, the 2000 Democratic vice presidential nominee, would have angered the party base but was generally considered qualified.
How McCain settled on Palin, whom he first met six months ago, remains a mystery outside his small inner circle of advisors.
She doesn't seem an ideal fit for a campaign that has focused intensely on foreign policy expertise and has attacked Obama for his relative lack of experience in that area.
At 44, she is three years younger than Obama and 21 years younger than Biden. She was elected governor in 2006 and formerly was mayor of Wasilla, Alaska, population 7,000 or so.
McCain apparently made his decision after a telephone conversation with Palin last weekend and a face-to-face meeting Thursday at his home in Sedona, Ariz.