Once again raising questions about the legitimacy of the 2016 elections, Donald Trump on Saturday night urged supporters at a rally in Pennsylvania to join together to check for voter fraud on Election Day in November, telling his audience to "watch and watch carefully" in "certain areas" of the Keystone State.

"You got to watch your polling booths, because I hear too many stories about Pennsylvania - certain areas," Trump said.

"I hear too many bad stories, and we can't lose an election because of you know what I'm talking about," Trump added.

Trump did not specify where those "certain areas" were in Pennsylvania that deserved additional attention on Election Day - but in the past, the GOP nominee and other Republicans have raised questions about voting patterns in overwhelmingly black areas of the city of Philadelphia.

In both 2008 and 2012, there were some precincts where John McCain and Mitt Romney did not get any votes; elections officials have denied anything was amiss.

"So, go and vote and then go check out areas," Trump said, "because a lot of bad things happen - and we don't want to lose for that reason."

It was a second straight day that Trump included such a warning to supporters in his stump speech - on Friday in Michigan, Trump told his backers to "make sure it's on the up and up," as he talked about Election Day.

It wasn't the first time that Trump himself brought up voter fraud in Pennsylvania - he did it in a Keystone State rally back in August.

At Saturday's rally in Lancaster County, Pennsylvania, Trump also went off his prepared speech text to again vent his frustrations about the first Presidential debate.

"Did anybody like Lester Holt?" Trump asked, naming the NBC News moderator, as the crowd booed.

Trump also again complained about his debate microphone, which the debate commission says had a problem inside the debate hall, but not on national television and radio broadcasts.

"How many people in this room think maybe that was done on purpose?" Trump asked to cheers.

The next Trump-Clinton debate is set for Sunday October 9.