U.S. Republican Party's presumed presidential nominee Mitt Romney (front) addresses a campaign event with his running mate Paul Ryan (1st R, front) in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)

Paul Ryan (R, front), running mate of U.S. Republican Party's presumed presidential nominee Mitt Romney (2nd L, front), addresses a campaign event in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)

U.S. Republican Party's presumed presidential nominee Mitt Romney (L, front) and his running mate Paul Ryan (R, front) wave during a campaign event in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)

U.S. Republican Party's presumed presidential nominee Mitt Romney and his running mate Paul Ryan attend a campaign event in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)

U.S. Republican Party's presumed presidential nominee Mitt Romney (L, front) shakes hands with his running mate Paul Ryan (R, front) during a campaign event in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)

U.S. Republican Party's presumed presidential nominee Mitt Romney (front) speaks during a campaign event together with his running mate Paul Ryan (2nd R, front) in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)

U.S. Republican Party's presumed presidential nominee Mitt Romney (front) addresses a campaign event with his running mate Paul Ryan (3rd L, front) in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)

U.S. Republican Party's presumed presidential nominee Mitt Romney (1st L) waves during a campaign event with his family and his running mate Paul Ryan (2nd R, front) in Manassas, Virginia, on Aug. 11, 2012. U.S. Republican Party's presumed presidential nominee Mitt Romney on Saturday formally announced U.S. Representative Paul Ryan of Wisconsin as his running mate, ending weeks of intense speculation. (Xinhua/Fang Zhe)