The selection of Ms. Palin, an evangelical Christian, was embraced by social conservative leaders, raising the possibility that Mr. McCain might have succeeded in reinvigorating a Republican base that was not enthusiastic about his candidacy. Democrats, however, dismissed the move as a bald attempt to attract women voters by reaching out to someone far too inexperienced to be a heartbeat from the presidency.

“She’s not from these parts, and she’s not from Washington, but when you get to know her, you’re going to be as impressed as I am,” Mr. McCain said as he introduced Ms. Palin to a crowd estimated by his campaign to be 15,000 at the Ervin J. Nutter Center at Wright State University.

Ms. Palin then took the stage with her husband, Todd, who owns a commercial fishing business. They were accompanied by four of their five children; she said the eldest child, a son, is in the Army, and he is heading to Iraq on Sept. 11.

She described herself as “just your average hockey mom,” who joined the P.T.A., was elected to the City Council and then served as mayor and as governor, adding that she did not get into government to take the safe course.

“A ship in harbor is safe, but that’s not why the ship is built,” Ms. Palin said, adding that she would “challenge the status quo to serve the common good.”

Ms. Palin praised the achievements of Senator Hillary Rodham Clinton, who lost a long and bitter primary race against Senator Obama, saying that she had left “18 million cracks” in the highest glass ceiling in the land.

Then, making an explicit appeal to Ms. Clinton’s disappointed supporters, she said, “It turns out that the women in America aren’t finished yet, and we can shatter that glass ceiling.”

Ms. Palin, a former mayor of the small town of Wasilla, an Anchorage suburb, and one-time beauty pageant queen, first rose to prominence as a whistle-blower uncovering ethical misconduct in state government.

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.

The choice of Ms. Palin stands in sharp contrast to the selection of the Democratic vice presidential nominee, Senator Joseph R. Biden Jr. of Delaware, a veteran lawmaker and 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 was elected Alaska’s chief executive after fighting off a comeback bid by a former Democratic governor. Her victory came after she had helped uncover misconduct in the administration of Gov. Frank Murkowski, whom she later trounced in a Republican primary.

Ms. Palin 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 did not.

But she differs with Mr. McCain on a controversial environmental issue that centers on her home state: she has been pushing for a new pipeline that would pump trillions of cubic feet of natural gas from the North Slope to the lower 48 states in the hope of delivering Alaska another economic boom. 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 pleased.

Although Ms. Palin had seemed like a long-shot, she was the subject of an effort on the Internet by conservatives to draft her as vice president.

She is known to religious conservatives for choosing not to have an abortion after learning that she was carrying a child with Down syndrome. “It is almost impossible to exaggerate how important that is to the conservative faith community,” said Ralph Reed, the former head of the Christian Coalition. She gave birth to her son, Trig, in April.