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The insult came as Trump blasted Senator Bernie Sanders for endorsing Clinton after losing a heated primary fight for the Democratic nomination.
"If he would have just not done anything - just go home, go to sleep, relax - he would have been a hero," Trump said at the rally in Mechanicsburg. "But he made a deal with the devil. She's the devil."
It's not the first time Trump has described Sanders' endorsement as a "deal with the devil." At rallies in Iowa and Colorado last week, the Republican nominee accused Sanders of "selling out to the devil" and said he "sold his soul to the devil," CNN reported.
But according to the Associated Press, Monday's rally marks the first time he's gone so far as to call Clinton herself "the devil."