"I'll look at it at the time," Trump said. "What I've seen is so bad."
An angry Clinton responded with incredulity to the highly controversial charge that questions the very health of US democracy.
"Let's be clear about what he's saying and what that means. He's denigrating and talking down our democracy," she said.
"I, for one, am appalled that somebody who is the nominee of one of our two major parties would take that kind of position."
Asked by the debate moderator whether he would concede if the results showed he lost to Clinton, Trump again demurred. "I'll keep you in suspense, okay?"
"This is horrifying," Clinton said.
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