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By John Whitesides ATLANTA (Reuters) - U.S. Vice President Joe Biden said on Thursday he is trying to determine whether he has the emotional energy to mount a White House bid in 2016 but cannot say yet whether he can do it. In his first extensive comments about a possible run for the Democratic presidential nomination, Biden said, "I can't look you straight in the eye now and say I know I can do it." Biden, 72, has been huddling with advisers for weeks to determine whether he will challenge Democratic front-runner Hillary Clinton, who has seen her favorability ratings plummet over her use of a private email server while working as the nation's top diplomat. "The most relevant factor in my decision is whether my family and I have the emotional energy to run," said Biden, who lost his son, Beau, to cancer earlier this year.