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"You've got to watch your polling booths, because I hear too many stories about Pennsylvania, certain areas," he said at a rally in Manheim, Pa.

"I hear too many stories, and we can't lose an election because — you know what I'm talking about. Go and vote and go check out areas, because a lot of bad things happen and we don't want to lose for that reason."

It's far from the first time Trump has raised the specter of voter fraud derailing the legitimacy election, or specifically raising concerns about fraud in Pennsylvania. But it underscores questions as to whether the Republican presidential nominee and his supporters will accept the results of the election if he doesn't win.

He's sent out emails to his supporters asking them to sign up to be Trump campaign election monitors. And he's questioned results from Philadelphia, implying that Republican votes weren't counted — a long-held rumor dismissed by PolitiFact

Political scientists have cautioned about the effects of partisan poll watchers, especially as polling shows Trump supporters are less confident about the accuracy of the election results.

"Trump’s poll watchers aren’t going to uncover any voter fraud," said Richard Cohen, the president of the Southern Poverty Law Center , in August.

"What they’re more likely to do, given the violence we’ve seen at Trump rallies and the support he’s getting from avowed white supremacists, is intimidate people and suppress the vote."