By PETER BAKER
The selection of Gov. Sarah Palin of Alaska proved quintessentially McCain — daring, hazardous and defiantly off-message. He demonstrated that he would not get boxed in by convention as he sought to put a woman next in line to the presidency for the first time. Yet in making such an unabashed bid for supporters of Senator Hillary Rodham Clinton, he risked undercutting his central case against Senator Barack Obama.
“Here’s what I’m worried about,” said Ed Rogers, a Republican lobbyist and former aide to Presidents Ronald Reagan and George Bush. “McCain had to protect his reputation as an opponent of status quo Washington. He had to pick someone with the shortest Washington résumé. He did that. He picked someone the right wing is going to be happy about. But it’s a gamble.”
“The question is,” Mr. Rogers continued, “what does it do to the argument that Obama’s not ready?”
The question is particularly acute for Mr. McCain, who turned 72 on Friday and would be the oldest person elected to a first term as president if he won in November. His campaign now needs to convince the public that it can imagine in the Oval Office a candidate who has spent just two years as governor of a state with a quarter of the population of Brooklyn.
But Ms. Palin, 44, brings clear assets to the ticket. The “gun-packing, hockey-playing woman,” as the Republican strategist Karl Rove described her, instantly bolstered Mr. McCain’s wobbly conservative base, which rejoiced over the selection of an anti-abortion evangelical Christian.
Her reputation as a reformer who took on her state party over corruption and wasteful spending could reinforce Mr. McCain’s own maverick appeal.
Her personal narrative as a working mother raising five children, including an infant with Down syndrome, with a husband who belongs to a union, might prove attractive to working-class voters in swing states who have been suspicious of Mr. Obama. And her presence on the ticket will allow Republicans to argue that Mr. Obama would not be the only one to break barriers if elected.
“He’s chosen a Washington outsider who will be an ally for him in shaking up the way things are done,” said Ron Nehring, chairman of the California Republican Party. “This is someone with solid conservative credentials but solid credentials as a reformer. And it’s clear after watching today’s event, no one is going to push Sarah Palin around — not Barack Obama and not Joe Biden,” the Democratic vice-presidential candidate.
In picking a running mate without deep experience but who would make history, Mr. McCain chose someone who in some ways resembles Mr. Obama. At the same time, by choosing Senator Joseph R. Biden Jr. of Delaware as his running mate, Mr. Obama tapped a longtime Washington hand with even more time in the Senate than Mr. McCain. Just as it might be harder for Mr. McCain to attack his opponent over his level of experience, it might be tougher for Mr. Obama to paint his rival as a creature of the capital.
The selection of Ms. Palin offered clues to how Mr. McCain would govern: holding deliberations to a tight circle of advisers, looking beyond the obvious options, taking risks and relishing surprise.
Yet if he disregarded more conventional prospects, like former Gov. Mitt Romney of Massachusetts, it might be that Ms. Palin was still the fallback from a more audacious decision that Mr. McCain ultimately eschewed.
In the end, he passed over two of his best political friends, Senator Joseph I. Lieberman, independent of Connecticut, and former Gov. Tom Ridge of Pennsylvania, knowing their support for abortion rights would inflame conservatives at next week’s Republican convention.
Ms. Palin has been a rising star on the right since she beat an incumbent governor in a Republican primary in 2006 and then a former Democratic governor in the general election. With an approval rating around 80 percent, she is among the most popular governors. But her success has come on a small stage. The 115,000 votes she received in winning the governor’s office two years ago barely eclipsed the 80,000 people who packed a football stadium in Denver on Thursday night to watch Mr. Obama’s acceptance speech.
Democratic strategists compared her selection to those of Geraldine A. Ferraro in 1984 or Dan Quayle in 1988, suggesting that the decision reflected desperation by Mr. McCain. “He feels a little like Walter Mondale,” said Jim Jordan, a Democratic political consultant. “He’s a respected Washington lifer who’s run into political forces that are bigger than himself. And he’s responded by making a decision that feels panicky.”
Some Republicans, though, distinguished her résumé from Mr. Obama’s by arguing that Ms. Palin’s executive experience as governor was more valuable than Mr. Obama’s legislative history. The “not ready” argument against Mr. Obama, they suggested, will focus more on judgment than pure experience. And they maintained that Ms. Palin would get the better of Mr. Biden, predicting that the veteran senator, who is known for his slashing attacks, would have a hard time not looking as though he was being condescending to a woman.
“In a way, McCain has set a trap on the experience argument,” said Scott Reed, who managed Bob Dole’s presidential campaign in 1996, “because if they start picking on her on experience, it’s going to backfire with women.”
A version of this article appeared in print on August 30, 2008, on page A1 of the New York edition.