DAYTON, Ohio — In a surprise move, Senator John McCain chose Gov. Sarah Palin of Alaska as his running mate on Friday, shaking up the political world at a time when his campaign has been trying to attract women, especially disaffected supporters of Senator Hillary Rodham Clinton, McCain officials confirmed.

In choosing Ms. Palin — a 44-year-old conservative Christian and self-described “hockey mom” who has been governor for less than two years — the McCain campaign reached far outside the Washington Beltway in an election in which the Democratic nominee, Senator Barack Obama, is running on a platform of change.

The selection amounted to a gamble that an infusion of new leadership — and the novelty of the Republican Party’s first female candidate for vice president — would more than compensate for the risk that Ms. Palin could undercut one of the McCain campaign’s central arguments, its claim that Mr. Obama is too inexperienced to be president.

Moreover, the choice of Ms. Palin is in stark contrast to the recent selection of the Democratic vice presidential nominee Senator Joseph R. Biden R. of Delaware, a respected veteran lawmaker who is chairman of Foreign Relations Committee.

But Ms. Palin ran as a change agent when she was elected as governor of Alaska in 2006, and in a move that might have appealed to Mr. McCain, she took intense criticism from members of her own party for turning the spotlight on the failures of Alaska Republicans, some of whom had been beset by corruption scandals.

She opposes abortion rights, which could help pacify social conservatives in a party whose members were wary as rumors swirled that Mr. McCain might pick a running mate who supports those rights. But she differs with Mr. McCain on a controversial environmental issue that centers on her home state: she supports drilling for oil in the Arctic National Wildlife Reserve. Mr. McCain’s opposition to drilling — even after he changed positions and began advocating for off-shore oil drilling — has upset many Republicans.

The choice of Ms. Palin was a closely guarded secret, and she flew under the political radar for months as Mr. McCain searched for a running mate. Much of the public discussion in recent days had focused on Mitt Romney, the former Massachusetts governor and Mr. McCain’s one-time rival for the Republican nomination; Gov. Tim Pawlenty of Minnesota; Tom Ridge, the former governor of Pennsylvania and Homeland Security secretary, and Senator Joseph I. Lieberman of Connecticut, the Democrat-turned-independent who was former Vice President Al Gore’s running mate in 2000. Social conservatives were relieved and highly pleased.

“They’re beyond ecstatic,” said Ralph Reed, the former head of the Christian Coalition. “This is a home run. She is a reformer governor who is solidly pro-life and a person of deep Christian faith. And she is really one of the bright shining new stars in the Republican firmament.”

Ms. Palin is known to conservatives for choosing not to have an abortion after learning two years ago that she was carrying a child with Down’s syndrome. “It is almost impossible to exaggerate how important that is to the conservative faith community,” Mr. Reed said.

Whether her selection will improve Mr. McCain’s appeal to women who had supported Mrs. Clinton is unclear. Both Mr. McCain and Ms. Palin oppose abortion rights, an important issue for some women. And a major theme of the Democratic convention that just concluded in Denver was both Hillary Rodham Clinton and former President Bill Clinton urging supporters to unite behind Mr. Obama.

The choice of Ms. Palin was reminiscent of George H. W. Bush’s selection of Dan Quayle, a young United States senator, as his running mate in 1988. The media and most in the Republican Party were caught unaware by the announcement of a figure relatively unknown outside Indiana.

Similarly, several of Mr. McCain’s outside advisers reacted with bewilderment that Ms. Palin was the choice, and one said that it would undercut one of Mr. McCain’s central criticisms of Senator Obama — that he is too inexperienced to be commander in chief.

“While it’s a dramatic and interesting choice, it would make the argument he’s making difficult to make,” said the adviser, who is close to the campaign.