DENVER - REPUBLICAN John McCain has made up his mind on who he will pick as his vice presidential running mate and is likely to announce his choice on Friday, a Republican official said on Wednesday.

The official, who has ties to the McCain campaign and spoke on condition of anonymity, said it was unclear who Mr McCain had selected. The choice is a closely guarded secret among Mr McCain's top campaign lieutenants.

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'They're looking at rolling it out Friday,' the Republican said.

The McCain campaign had no comment.

Mr McCain is to be in Dayton, Ohio, on Friday, the day after the Democratic national convention ends.

A Friday announcement would be aimed at blunting momentum that Democratic presidential nominee Barack Obama would be expected to gain from his convention in Denver.

Mr McCain and his aides have debated whether to pick a solidly Republican running mate such as former Massachusetts Gov Mitt Romney, who has campaigned hard for Mr McCain after losing to him in the Republican primary fight, or go with a less traditional choice such as independent Democratic Sen Joe Lieberman, a McCain booster.

Mr Lieberman was Democrat Al Gore's running mate in the 2000 campaign, and some Republicans fear picking Lieberman in an effort to appeal to independent voters would alienate core conservatives and make Mr McCain's nominating convention next week in St Paul, Minnesota, a disaster.

Other candidates in the mix are believed to have been Minnesota Gov Tim Pawlenty, former EBay chief executive Meg Whitman and Texas Republican Sen. Kay Bailey Hutchison, among others. -- REUTERS