Speaking at a rally in a high school gym in Pennsylvania, Mr Trump attacked Bernie Sanders for capitulating to Mrs Clinton in the Democratic race.
"He made a deal with the devil," the tycoon said. "She's the devil."
Democrats and Republicans alike have condemned Mr Trump for his remarks about a US Muslim soldier's parents.
Former Republican candidate John McCain became the latest senior Republican to criticise Mr Trump for his attack on the parents of US Army Capt Humayun Khan, who was killed by a car bomb in 2004 in Iraq, at the age of 27.
Senator McCain, a veteran of the Vietnam War, said in a strongly worded statement that Mr Trump did not have "unfettered licence to defame the best among us".
The soldier's parents, Khizr Khan and his wife Ghazala, told the BBC it was time to stand up to Mr Trump but he accused them of "viciously" attacking him.
Mr Trump had caused controversy by suggesting Ghazala Khan had been prevented from speaking alongside her husband at the Democratic convention last week.
The Republican candidate had already described the decision of Mr Sanders to back Mrs Clinton at the convention as a "deal with the devil".
However, his remarks on Monday evening in the gym in Mechanicsburg, Pennsylvania, were the first time he went so far as to specifically equate Mrs Clinton with the devil.
In another development, American billionaire businessman Warren Buffett challenged Mr Trump to release his tax returns.
Mr Trump has said that they cannot be made public until the financial authorities have completed an audit.
But Mr Buffett said there were no rules against showing tax returns and allowing people to ask questions about them. Speaking at a rally in support of Mrs Clinton, Mr Buffett said he was under audit as well, adding he was prepared to meet Mr Trump "any place, any time" to go over each other's tax returns.