SIOUX FALLS, S.D. - Former Sen. George McGovern, the 1972 Democratic presidential candidate and an early backer of Hillary Rodham Clinton, joined a noisy rally for Barack Obama on Friday night, describing the Illinois senator as a "ripple of hope" who can win the White House. McGovern has shifted his allegiance to Obama and suggested it's time for Clinton to pull the plug. He offered an explanation to a cheering crowd of 6,500. "Last October, I endorsed Senator Hillary Clinton," said McGovern. "She and her remarkable husband, President Clinton, way back 36 years ago worked their hearts out for me...