DENVER - John McCain tapped little-known Alaska Gov. Sarah Palin to be his vice presidential running mate on Friday in a startling selection on the eve of the Republican National Convention.

Two senior campaign officials disclosed McCain's decision a few hours before the Republican presidential nominee-to-be and his newly-minted running mate appeared at a rally in swing-state Ohio.

Palin, like McCain, is a conservative with a maverick streak who has shown a willingness to clash with others in her own party. A self-styled hockey mom and political reformer, she has been governor of her state less than two years.

Palin's selection shocked numerous Republican officials.

At 44, Palin is a generation younger that Sen. Joseph Biden of Delaware, who is Barack Obama's running mate on the Democratic ticket.

She is three years Obama's junior, as well — and McCain has made much in recent weeks of Obama's relative lack of experience in foreign policy and defense matters.

In making his pick, McCain passed over several more prominent prospects who had figured in speculation for months — Minnesota Gov. Tim Pawlenty, former Massachusetts Gov. Mitt Romney and former Pennsylvania Gov. Tom Ridge among them.

Palin flew overnight to an airport in Ohio near Dayton, and even as she awaited her formal introduction, some aides said they had believed she was at home in Alaska.

She is a former mayor of Wasilla who became governor of her state in December, 2006 after ousting a governor of her own party in a primary and then dispatching a former governor in the general election.

More recently, she has come under the scrutiny of an investigation by the Republican-controlled legislature into the possibility that she ordered the dismissal of Alaska's public safety commissioner because he would not fire her former brother-in-law as a state trooper.

The timing of McCain's selection appeared designed to limit any political gain Obama yields from his own convention, which ended Thursday night with his nominating acceptance speech before an estimated 84,000 in Invesco Field in Colorado.

Public opinion polls show a close race between Obama and McCain, and with scarcely two months remaining until the election, neither contender can allow the other to jump out to a big post-convention lead.

McCain has had months to consider his choice, and has made it clear to reporters that one of his overriding goals was to avoid a situation like the one in 1988, when Dan Quayle was thrown into a national campaign with little preparation.

Palin has a long history of run-ins with the Alaska GOP hierarchy, giving her genuine maverick status and reformer credentials that could complement McCain's image.

Two years ago, she ousted the state's Republican incumbent governor, Frank Murkowski in the primary, despite having little money and little establishment backing.

She has also distanced herself from two senior Republican office-holders, sen. Ted Stevens and Rep. Don young. Both men are under federal corruption investigations.

She had earned stripes — and enmity — after Murkowski made her head of the Alaska Oil and Gas Conservation Commission. From that post, she exposed ethical violations by the state GOP chairman, also a fellow commissioner.

She and her husband Todd Palin, have five children. The latest, a baby, was born with Down syndrome.