Just as polls in key battleground states show him lagging behind rival Hillary Clinton, Donald Trump stoked fears of voter fraud in Pennsylvania Friday, pushing his supporters to police polling places on Election Day for any "cheating."

"The only way we can lose in my opinion -- I really mean this, Pennsylvania -- is if cheating goes on," Trump told rally attendees Friday evening during an event in Altoona. "I really believe it."

Trump, who attended college at the University of Pennsylvania, went on to say that he knew the state "well" and that he had been "studying it."

Get Breaking News Delivered to Your Inbox

"Because I looked at Erie and it was the same thing as this. We have some great people here. Some great leaders here of the Republican party, and they're very concerned about that," said Trump, who was introduced at the rally by Republican National Committee chair Reince Priebus. "And that's the way we can lose the state."

The GOP nominee then called for further policing in the state on Nov. 8, the day of the general election.

"We have to call up law enforcement, and we have to have the sheriffs and the police chiefs and everybody watching," Trump said. "Because if we get cheated out of this election, if we get cheated out of a win in Pennsylvania, which is such a vital state, especially when I know what's happening here, folks. I know. She can't beat what's happening here."

Trump continued, urging his supporters to personally monitor polling places for voter fraud.

"The only way they can beat it in my opinion -- and I mean this 100 percent -- if in certain sections of the state, they cheat, okay?" he said. "So I hope you people can sort of -- not just vote on the 8th -- go around and look and watch other polling places and make sure that it's 100 percent fine."

On Friday, Trump's campaign also rolled out a new volunteer sign-up feature on his website, asking backers to become a "Trump Election Observer." The page reads: "Help Me Stop Crooked Hillary From Rigging This Election!"

At the rally, the billionaire expressed shock that Pennsylvania did not have "voter identification."

"Because without voter identification, which is shocking, shocking that you don't have it. ... Many states it gets approved and some states they don't get approved," Trump said.

The state, along with 19 others, has no ID requirement for polling places. In 2014, a Pennsylvania judge struck down a law requiring voters to show photo identification, noting in his ruling at the time that "voting laws are designed to assure a free and fair election; the Voter ID Law does not further this goal."

In-person voter fraud, which identification laws are meant to prevent, is an extremely rare occurrence, according to several studies. One analysis reported by the Washington Post and conducted by Loyola Law School professor Justin Levitt looked at 14 years of voting (from 2000 to 2014) and found approximately 241 possibly fraudulent ballots out of nearly a billion ballots cast.

Pennsylvania itself has been the target of voter fraud accusations before, like in 2012 when then-GOP nominee Mitt Romney received zero votes in certain areas of Philadelphia. But as the Philadelphia Inquirer reported at the time, "the unanimous support for [President] Obama in these Philadelphia neighborhoods -- clustered in almost exclusively black sections of West and North Philadelphia -- fertilizes fears of fraud, despite little hard evidence." Mr. Obama, in 2012, won over 90 percent of the African-American vote nationwide.

Trump discussed the 2012 Philadelphia voter results in an interview with Fox News host Sean Hannity in early August, sparking Twitter backlash:

1. I'm an inspector of elections for a Philly voting division. Independent but was a Republican as recently as June. https://t.co/pd82mOkEKh — Ryan Godfrey (@rgodfrey) August 7, 2016

3. Claim that 59 divisions in Philadelphia engaged in electoral fraud in 2012 because no votes for Romney is absurd & personally insulting. — Ryan Godfrey (@rgodfrey) August 7, 2016



Pennsylvania has also leaned Democratic in the last two presidential election cycles, with Mr. Obama winning 52 percent to Romney's 46.8 percent in 2012. In 2008, Mr. Obama triumphed over John McCain 54.5 percent to 44.2 percent.

In this year's general election race, a RealClearPolitics average of Pennsylvania polls finds Clinton leading Trump 49 percent to 40 percent.