Residents cast their vote at the Shoesmith Elementary School near President Barack Obama`s house, in southern Chicago, Illinois, the United States, Nov. 6, 2012. The quadrennial U.S. presidential elections kicked off Tuesday. (Xinhua/Zhang Jun)

　　Residents cast their vote at the Shoesmith Elementary School near President Barack Obama`s house, in southern Chicago, Illinois, the United States, Nov. 6, 2012. The quadrennial U.S. presidential elections kicked off Tuesday. (Xinhua/Zhang Jun)

　　A voter shows his sticker "I Voted!" after voting at a polling station in San Francisco, Nov. 6, 2012. The quadrennial U.S. presidential elections kicked off Tuesday. (Xinhua/Liu Yilin)

　　A voter walks into a polling station for voting in San Francisco, Nov. 6, 2012. The quadrennial U.S. presidential elections kicked off Tuesday. (Xinhua/Liu Yilin)

　　People queue up to vote at Boston public library in Boston, the United States, on Nov. 6, 2012. The quadrennial U.S. presidential elections kicked off Tuesday. (Xinhua/Shen Hong)