Democrat Barack Obama prolonged the suspense on his vice presidential pick yesterday, saying he has chosen an independent voice who could be an effective governing partner - but not revealing the name.

Sentaor Obama has dragged out the announcement of a No. 2 for maximum political impact, but is running out of time. The Democratic nominating convention opens on Monday, with the vice-presidential candidate accepting the nomination on Wednesday.

"I'm pretty disciplined on this," he said on CBS's The Early Show when asked about the choice.

Mr Obama, 47, a first-term Illinois senator, said he had searched for a running mate who will be prepared to step in as President and one who could help him govern.

"The third criterion for me, I think, was independence. I want somebody who is going to challenge my thinking and not simply be a yes person when it comes to policy-making," he said.

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