A formal announcement was expected within a few hours at a campaign rally in swing-state Ohio.
Palin, 44, is a self-styled hockey mom and political reformer who has been governor of her state less than two years.
Palin's selection was a stunning surprise, as McCain passed over many other better known prospects, some of whom had been the subject of intense speculation for weeks or months.
At 44, she 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.
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