Donald Trump is threatening to upend a fundamental pillar of American democracy.
He's refusing to say that he will accept the results of next month's election if he loses – warning voters the election will be “rigged.”
The Republican National Committee immediately distanced itself from his statement.
Speaking in a live TV debate in Las Vegas, Mr Trump again accused the campaign of being rigged.
Democrat candidate Hillary Clinton's described his decision as “horrifying”.
Hillary Clinton says Mr Trump will deny anything that doesn't go his way
