Republican John McCain picked Alaska Governor Sarah Palin as his surprise choice for running mate on Friday, adding a political unknown to the presidential ticket who could help him appeal to women voters.

Palin, 44, will appear with McCain later today in a rally in Dayton, Ohio, as the Arizona senator grabs the political focus away from Democratic rival Barack Obama one day after Obama accepted his party's presidential nomination.

"Governor Palin is a tough executive who has demonstrated during her time in office that she is ready to be president," McCain said in a statement announcing the choice.

"She has brought Republicans and Democrats together within her administration and has a record of delivering on the change and reform that we need in Washington," the statement said.

McCain and Palin will face Obama and his running mate, Joe Biden, in the November 4 presidential election.

The choice followed days of speculation about McCain's choice, with most of the better-known contenders like former Massachusetts Governor Mitt Romney slowly eliminated over the last 24 hours.

Palin is a conservative first-term governor of Alaska with strong anti-abortion views, a record of reform and fiscal conservatism and an outsider's perspective on Washington. She is also an avid sportswoman who would bring youth and vitality to the ticket.

McCain turns 72 on Friday and would be the oldest person to take office for a first term in the White House if elected.

Palin is almost unknown nationally, which could hurt McCain's argument that Obama, a first-term senator from Illinois, is too inexperienced to handle the White House.

But she could help him appeal to disaffected supporters of Democrat Hillary Clinton, who lost a bruising primary to Obama.

In his acceptance speech, Obama directly attacked McCain and linked him to what he called the failed Republican policies of President George W. Bush, promising to reverse eight years of economic failure and restore the standing of the United States in the world.

Palin, who until two years ago was mayor of the town of Wasilla, built a reputation as a reformer in a state that recently has been hit with corruption scandals. Elected in 2006, she is Alaska's first woman governor.

She and her husband, Todd, have five children, ranging in age from 18 years to 4 months. The youngest has Down's Syndrome.

"She brings an excitement on the part of the Republican base. The strong conservatives really like her and you have to activate the base," said Dennis Goldford, a professor of politics at Drake University in Des Moines.

"He is also clearly trying to attract Clinton supporters who want to see a female in the VP's office ... On the down side there is no way in the world she passes the commander-in-chief test."

If elected, Palin would be the first woman US vice president, adding another historic element to a presidential race that has been filled with firsts. Obama, 47, is the first black nominee of a major US political party.

McCain and Republicans open their national convention on Monday.

Obama and McCain are running neck-and-neck in polls and McCain has quickly grabbed back the political limelight with the choice of Palin.

The two will appear together at a rally before about 10,000 supporters later today in a state, Ohio, that is a crucial battleground state in November's election.