Every living American president, except one, is preparing to attend Tuesday’s memorial for former South African President Nelson Mandela. President Obama and former presidents George W. Bush, Bill Clinton and Jimmy Carter are expected to join dozens of other heads of state at the service in Johannesburg. Obama will be leaving the White House later this Monday morning. The only U.S. president who is not going to attend is George H.W. Bush. The elder Bush has been in poor health.