FORT MYERS, Fla. — His party in chaos, a surging Donald Trump called on mainstream Republicans to unify behind his candidacy on Wednesday as his White House rivals seized on their last best opportunity to block the billionaire businessman from building an insurmountable delegate lead in two key states. The often-brash Trump softened his tone, at least temporarily, hours after securing another three primary victories, praising House Speaker Paul Ryan as a man he respects and encouraging Mitt Romney to promote party unity. The comments marked a sharp reversal in tone, if not substance, from a week earlier when Trump hurled insults at his party's 2012 presidential ticket.