From a BBC report on all this: In a podcast last week, long-time Trump advisor Roger Stone said that if the election results in November don't match opinion polls, the Republican nominee should challenge the validity of the election and warned that the unrest could end in a "bloodbath".
"If there's voter fraud, this election will be illegitimate," he said. "The election of the winner will be illegitimate, we will have a constitutional crisis, widespread civil disobedience, and the government will no longer be the government."
Such upheaval may be painfully familiar to those living in the world's less established democracies. It would be uncharted territory for the United States.

Worrying stuff.