In a move that startled the political world, Sen. has chosen Alaska Governor Sarah Palin to be his running mate.
"This is John McCain the fighter pilot," said CBS News chief Washington correspondent Bob Schieffer when he heard the news. "He is willing to take a risk and put it all up on the line. I think the Republican base will be pleased by this."
Palin, 44, will be the first female Republican vice-presidential candidate. A first-term governor and former beauty queen, Palin is known for her dynamic personality. She is a lifelong member of the National Rifle Association and is anti-abortion rights.
Palin signed into law an aggressive ethics reform package and is at the forefront of the oil drilling debate in Alaska. She faces a state probe into whether she tried to get a former brother-in-law fired from his law enforcement job.
"It definitely reflects a decision on two fronts," said CBS News political analyst Dan Bartlett, a former adviser to President Bush. "One that with the departure of Hillary Clinton from the race that this is an opportunity for McCain to make even more headway to a key voting block."
"Second, they're aiming to reinforce message of reform. Palin is someone who has made a mark on both the state of Alaska and nationally as someone who has taken on ethics reform boldly," Bartlett added. "The McCain campaign seems to believe those two qualities will more than make up for her lack of national exposure."
In picking Palin, McCain passed on more traditional picks, such as former Massachusetts governor Mitt Romney and Minnesota governor Tim Pawlenty, both of whom were thought to be on the Republican's shortlist.
Palin is the youngest and first female governor of Alaska. She has five children.
A formal announcement is expected in Dayton, Ohio, at noon ET.
Palin flew last night from Anchorage to Ohio. Her trip was kept so secretive that as early as Friday morning, members of her staff said that she was still 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 Democratic nominee Barack 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.