Former US President Barack Obama says his successor, Donald Trump, is deeply unfit for the presidency. He is urged Americans to vote for Joe Biden, for the survival of American democracy. Obama's words echo those of his wife, Michelle, who said Trump has proven he is incapable of meeting the moment. They have spoken at the Democratic National Convention. Biden was formally nominated on Tuesday night to take on Trump in the November presidential election. Senator Kamala Harris is his choice for vice-president.