Donald Trump called his Democratic rival 'the devil' on Monday in a high-profile escalation of his campaign rhetoric – at precisely the moment he needed to change the subject.

Facing a four-day-long scandal over his criticism of the father of a Muslim soldier who died in combat, the Republican presidential nominee shifted gears.

Sen. Bernie Sanders, Trump told an overflow crowd at a high school in Mechanicsburg, Pennsylvania, 'made a deal with the devil' when he endorsed Clinton at last week's Democratic National Convention.

'She's the devil!' he added.

Trump has hinted at the insult before, saying Friday in Colorado that 'Bernie blew it. He sold his soul to the devil. He did.'

But Trump had previously stopped short of declaring that Hillary Clinton is, in fact, the Prince – or Princess – of Darkness.

By rallying around Clinton instead of standing on principle, the billionaire said, Sanders 'made a bad deal. He should have not made a deal. He would have gone down as [having] done something really important.'

'Once he made that deal, and believe me, he has buyer's remorse.'

'If he would have just not done anything – just go home, go to sleep, relax – he would have been a hero,' he said. 'But he made a deal with the devil. She's the devil! He made a deal with the devil. It's true.'