The foe-to-friend story will be at the center of the Obama-Clinton show in Charlotte, North Carolina, aides to both say. In his remarks, the president will act as a character witness for his former secretary of state, who is struggling to convince voters of her trustworthiness and honesty. There is no better politician to testify on her behalf, many Democrats believe, than the man who once counted himself among the Clinton skeptics but came around to be one of her biggest boosters. 'I think that he can be very helpful, particularly with Democratic voters and some independent voters who have...