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.