DENVER — Senator McCain’s choice of a first-term governor of Alaska, Sarah Palin, as his running mate brings the Republican ticket the youth and historic aspects that Senator Obama showcased at the Democratic convention just concluded here — and also some of the inexperience.

Ms. Palin, 44, became governor 20 months ago and would be the first woman ever to run on a national Republican ticket. A self-described “average hockey mom” and an opponent of abortion and gun control, she is considered solidly conservative and has drawn praise for her success in government in Alaska. She is unknown to the broad electorate and a newcomer to the national stage.

Before her election to the statehouse in 2006, Ms. Palin served as mayor of Wasilla, a suburb of Anchorage. She is a mother of five children, including an infant who was born with Down syndrome in April and an older child who is in the army and is soon to deploy to Iraq.

The choice reshapes the dynamic of a race that has pitted, in Mr. McCain, a 72-year-old fixture of Washington, against Senator Obama, 47, who has served in the Senate for less than three years. It signals a bid by Mr. McCain to increase his share of the women’s vote and, in particular, woo disaffected supporters of Senator Clinton who have yearned to see a woman in top national office.

Mr. McCain introduced Ms. Palin at a large rally in Dayton, Ohio, where the soon-to-be Republican presidential nominee told a cheering crowd estimated at 15,000 that he had been looking for a running mate “who can best help me shake up Washington.”

He heralded Ms. Palin’s “grit” in standing up to special interests in Alaska, and lauded her as a union member and woman of the working class. “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. “She knows where she comes from, and she knows who she works for. She stands up for what’s right and she doesn’t let anyone tell her to sit down.”

Ms. Palin appeared in the hall moments later, accompanied by her husband, Todd, and four of her children. Mr. McCain’s wife, Cindy, and his daughter Meghan were also on stage. The newly minted running mates exchanged a brief hug and Ms. Palin took the podium. She thanked Mr. McCain for having the “confidence” to trust her and said she was honored to accept the invitation.

“I know that it will demand the best that I have to give, and I promise nothing less,” she said.

Briefly describing her family, she said her husband was a commercial fisherman, a production operator in the oil industry, a member of the United Steel Workers union, and a “champion snow machine racer.” Her eldest son, Track, enlisted in the army last year and will be deployed to Iraq on September 11.

After touting Mr. McCain’s record on national security, Ms. Palin paid tribute to the first woman to run on a national ticket, Geraldine Ferraro in 1984, and made an explicit appeal to supporters of Senator Clinton and women more broadly. “It was rightly noted in Denver this week that Hillary left 18 million cracks in the highest, hardest glass ceiling in America,” she said. “But it turns out the women of America aren’t finished yet, and we can shatter that glass ceiling once and for all.”

Ms. Palin made her name in Alaska by taking on the establishment. She earned praise for exposing ethical violations by party leaders as head of the state Oil and Gas Conservation Commission. She defeated the incumbent Republican governor, Frank Murkowski, in a primary in 2006 before going on to win the general election.

While in office, she has distanced herself from Senator Stevens, a state luminary who is facing a fall trial on corruption-related charges. But she has run into her own problems more recently amid reports that she pushed for the firing of a state trooper involved in a custody battle with the governor’s sister.

Ms. Palin should add a sense of excitement to the Republican National Convention beginning in St. Paul next week, especially among conservatives who had feared that Mr. McCain would select a running mate who supports abortion rights, such as Governor Ridge of Pennsylvania or Senator Lieberman of Connecticut, who was the Democratic vice presidential nominee in 2000.

At the same time, the pick of Ms. Palin could undercut Mr. McCain’s attempts to focus attention on Mr. Obama’s inexperience, and the campaign will have to answer the question of whether she is ready to step in as president if something happens to Mr. McCain, who would be the oldest man ever elected to a first term in the White House.

While Ms. Palin had been mentioned as a long-shot possibility in the weeks after Mr. McCain secured the Republican nomination in March, speculation had centered in recent days on his former rival for the nomination, Mitt Romney, Governor Pawlenty of Minnesota, and Mr. Lieberman.

“I didn’t see it coming,” a prominent conservative activist who is president of Americans for Tax Reform, Grover Norquist, said.
Mr. Norquist praised Ms. Palin. “From a tax perspective, it’s very good. From a Reagan Republican perspective, it’s perfect,” he said. “I don’t think there’s a part of the party who won’t be excited.”

The chairman of the American Conservative Union, David Keene, called the selection “great news for conservatives.” “I predict any conservatives who have been lukewarm thus far in their support for the McCain candidacy will work their hearts out between now and November for the McCain-Palin ticket,” he said in a statement.

President Bush issued a statement praising Mr. McCain’s choice as “exciting” and calling Ms. Palin “a proven reformer who is a wise steward of taxpayer dollars and champion for accountability in government.”

The Obama campaign immediately questioned Ms. Palin’s readiness for the job. “Today John McCain put the former mayor of a town of 9,000 with zero foreign policy experience a heartbeat away from the presidency,” a spokesman, Bill Burton said. “Governor Palin shares John McCain’s commitment to overturning Roe v. Wade, the agenda of Big Oil and continuing George Bush’s failed economic policies — that’s not the change we need, it’s just more of the same.”

After that statement was criticized for belittling a woman governor’s accomplishments and patronizing small-town America, the campaign released a separate and considerably more cordial statement from Mr. Obama and Senator Biden congratulating Ms. Palin as a groundbreaking choice. “It is yet another encouraging sign that old barriers are falling in our politics,” they said. “While we obviously have differences over how best to lead this country forward, Governor Palin is an admirable person and will add a compelling new voice to this campaign.”