MECHANICSBURG: On the campaign stump Donald Trump has opened August by moving from 'crooked Hillary' to 'the devil.'

Speaking in Pennsylvania on Monday, the billionaire real estate mogul derided Bernie Sanders' capitulation in the Democratic primary race and Sanders' decision to support Clinton.

Trump said of Sanders: "He made a deal with the devil. She's the devil."

Trump in recent days has taken to categorising the the Clinton-Sanders understanding as a "deal with the devil" but this was the first time that he went so far as to specifically equate Clinton with Lucifer.

Trump's supporters packed a Mechanicsburg, Pennsylvania high school gymnasium and thousands more were left outside or forced to watch in a spillover room.