McCain was to introduce his choice at an event on Friday at midday in Dayton, Ohio.

Palin, 44, was a relatively surprising pick, but one aimed at appealing to women voters who might have been disillusioned by Democratic Pesidential candidate Barack Obama's decision to pick Biden as his No. 2 instead of Democratic Sen. Hillary Clinton of New York.

McCain and Palin, face the Democratic nominee Barack Obama and his No. 2, veteran Sen. Joe Biden, in the Nov. 4 election.

"She will be his partner in reforming Washington," said the senior McCain official, who spoke on condition of anonymity.

By announcing his pick on Friday, McCain was hoping to divert attention from the Democratic convention in Denver, which concluded on Thursday night with a speech by Obama. The Democratic candidate opened a broad assault on McCain, accusing him of following the policies of unpopular President George W Bush.

"This is a bold choice of a strong conservative who is a reformer and will be greeted by the delegates in St. Paul with enthusiasm," said Republican strategist Scott Reed.