Donald Trump has dismissed Barack Obama's time in the White House as a "disaster" after the US president said he was not fit to succeed him. "He's been weak, he's been ineffective," Republican candidate Mr Trump said of Mr Obama in a Fox News interview on Tuesday. He mocked Mr Obama's handling of the crisis with Russia over Ukraine. Earlier, Mr Obama said Mr Trump was unfit to be president, and questioned why his party still backed him. "There has to come a point at...