In the beginning, Senator Barack Obama was not entirely sold on Senator Joseph R. Biden Jr. And Mr. Biden told friends that he was pessimistic of his chances of becoming Mr. Obama's Democratic running mate.
"It's a very personal decision," said David Axelrod, the campaign's chief strategist, in a brief interview Saturday. "He approached it in a very serious, sober and reasoned way."
Mr. Obama reached the decision about 10 days ago while on a weeklong vacation to Hawaii. That week, Mr. Biden's strengths in foreign policy were highlighted by the conflict between Russia and Georgia, giving his prospects a further boost. Associates of the other main possibility on Mr. Obama's list, Senator Evan Bayh of Indiana, said Mr. Obama cited the situation in Georgia in breaking the news to Mr. Bayh late last week that he had chosen Mr. Biden.