(CNN) -- Sen. John McCain announced Alaska Gov. Sarah Palin as his vice presidential candidate Friday, calling her "the running mate who can best help me shake up Washington."
"She's exactly who this country needs to help me fight the same old Washington politics of me first and country second," the presumptive Republican nominee said at a Dayton, Ohio, rally of about 15,000 supporters, who welcomed the surprise pick of the relatively unknown politician with cheers and flags.
"She's got the grit, integrity, good sense and fierce devotion to the common good that is exactly what we need in Washington today," McCain said.
Palin, 44, described herself as a fighter against corruption and a bipartisan reformer in her first appearance as a candidate for vice president, an office she said she never expected to seek.
"I was just your average hockey mom in Alaska" before getting involved in politics, she said. "When I found corruption there, I fought it hard and brought the offenders to account."
Palin told the crowd, "To have been chosen brings a great challenge. I know that it will demand the best that I have to give, and I promise nothing less." Watch Palin say she's honored to be picked »
Palin is a first-term governor who unseated incumbent Gov. Frank Murkowski in the Republican primary in 2006 and went on to defeat former Gov. Tony Knowles, a Democrat, in the general election.
Campaigning for governor, she described herself as a "conservative Republican, a firm believer in free-market capitalism" and "a lifelong Alaskan who grew up hunting and fishing."
She boasted of eliminating taxes as mayor in her hometown of Wasilla, Alaska. She also was a City Council member in the town and was chairwoman of Alaska's Oil and Gas Conservation Commission, which regulates oil and gas resources.
Palin will be the first woman to be nominated for vice president as a Republican and only the second to run for vice president on a major party ticket, after Democrat Geraldine Ferraro in 1984.
Palin also will be the first Alaskan to be on the ticket of either party.
She described herself Friday as "commander in chief of the Alaska National Guard" and noted that her oldest son, Track, is a U.S. soldier scheduled to deploy soon to Iraq.
But there also are aspects of her biography that make her unusual for a Republican candidate. McCain introduced her as a former union member and the wife of a union member, a nod to Ohio's strong organized-labor culture. iReport.com: What do you think of McCain's VP pick?
Wasilla mayor, 1996-2002
Elected governor of Alaska, 2006
Sources: Almanac of American Politics, CNN
Palin made her name in part by backing tough ethical standards for politicians. During the first legislative session after her election as governor, her administration passed a state ethics law overhaul.
Nevertheless, she is under investigation for her firing of a state official, Public Safety Commissioner Walt Monegan. She has been instructed to hand over documents and recordings of telephone conversations as part of the inquiry, which grew out of allegations that she sacked Monegan for refusing to fire her former brother-in-law from the state police.
She has denied wrongdoing.
But Palin acknowledged that a member of her staff made a call to a trooper in which the staffer suggested that he was speaking for the governor.
Palin has admitted that the call could be interpreted as pressure to fire state trooper Mike Wooten, who was locked in a child-custody battle with Palin's sister. She suspended the staffer who made the call.
McCain apparently is making a concerted effort to reach out to former supporters of Sen. Hillary Clinton who may be unhappy with the choice of Sen. Barack Obama as the Democratic nominee. Watch McCain praise Palin »
"Hillary left 18 million cracks in the highest, hardest glass ceiling in America. But it turns out the women of America aren't finished yet, and we can shatter that glass ceiling once and for all," Palin said in her speech.
The remark brought a mixed reaction from the Republican crowd; some women cheered, but there was also some low-pitched groaning and booing.
Democratic leaders, including the party's presidential nominee Barack Obama and Sen. Hillary Clinton, also issued mixed reactions to Palin's announcement.
"We should all be proud of Gov. Sarah Palin's historic nomination, and I congratulate her and Sen. McCain. While their policies would take America in the wrong direction, Gov. Palin will add an important new voice to the debate," Clinton said.
"Ultimately John McCain is at the top of the ticket. As I indicated in my speech last night, I think that he wants to take the country in the wrong direction. I'm assuming Gov. Palin agrees with him in in his policies," said Obama, who called Palin later in the day to congratulate her.
"But the fact that she's been nominated, I think, or will soon be nominated, I think is one more indicator of this country moving forward," said Obama. "I congratulate her and look forward to a vigorous debate."
His campaign issued a more pointed response.
"Today, John McCain put the former mayor of a town of 9,000 with zero foreign policy experience a heartbeat away from the presidency. Gov. 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," Obama spokeswoman Adrianne March said.
Rep. Thad McCotter, a Michigan Republican, said Palin has more executive experience than the entire Democratic ticket.
Most of the speculation about McCain's choice had focused on former Massachusetts Gov. Mitt Romney, Minnesota Gov. Tim Pawlenty and Sen. Joseph Lieberman, an independent from Connecticut and 2000 Democratic vice presidential nominee. Sen. Kay Bailey Hutchison of Texas also had been mentioned as a possible long shot if McCain were to seek a woman.
Palin is the youngest person elected governor of Alaska and the first woman to hold the job. iReport.com: McCain's pick called "a stroke of genius"
She was dubbed "Sarah Barracuda" by opponents when she was mayor in Wasilla, resurrecting a nickname she earned as a state champion high school basketball player, according to the Almanac of American Politics.
She is married to Todd Palin, an oil production operator on Alaska's North Slope. They have five children. Her youngest, Trig, was diagnosed in utero with Down syndrome.
Palin has focused on energy policy during her short stint in office, and she is known for her support of drilling in the Arctic National Wildlife Refuge, a position McCain opposes but many grassroots Republicans support.