WASHINGTON — Senator Barack Obama introduced Senator Joseph R. Biden Jr. as his running mate at a boisterous rally in Springfield, Ill., on Saturday, a choice that strengthens the Democratic ticket’s credentials on foreign policy and provides Mr. Obama a combative partner as he heads into the fight with Senator John McCain.

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In Mr. Biden, Mr. Obama selected a six-term senator best known for his expertise on foreign affairs — Mr. Biden spent last weekend in Georgia as that nation engaged in a tense confrontation with Russia — but also for his skills at political combat. Mr. Obama passed over other candidates who might have brought him a state or reinforced the message of change that has been central to his candidacy.

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