CHICAGO (AP) — Warning of a difficult fight ahead, former President Barack Obama and Michelle Obama on Tuesday called on the nation to embrace Kamala Harris in urgent messages to the Democratic National Convention that were at times both hopeful and combative. “America, hope is making a comeback,” the former first lady declared. She then tore into Republican Donald Trump, a sharp shift from the 2016 convention speech in which she told her party, “When they go low, we go high.” “His limited and narrow view of the world made him feel threatened by the existence of two hardworking, highly educate…