U.S. Republican presidential nominee Donald Trump holds a campaign rally in Delaware, Ohio, Oct. 20, 2016. Jonathan Ernst/Reuters

Donald Trump has constantly expressed concerns about the results of the upcoming presidential election. Late Monday, officials who run the voting systems also raised concerns over a possible voting fraud.

In a report, TMZ pointed out that there are some states that do not require voters to present any ID before casting a ballot. Anyone can cast a vote if the person's signature matches the one on the voter registration form.

Nick LaLota, Suffolk County NY Board of Elections commissioner, told TMZ that signature verification compromises the integrity of the voting process as it will be hard to know who votes when they shouldn't with hundred thousands of people flooding to poll booths to cast their vote on Nov. 8.

Marcy Crawford, deputy commissioner of the Board of Elections for Allegany County, also said that voter fraud is a real concern in her county.

According to John Arntz, director of elections in San Francisco, voters show up at the polling stations and a verbal address verification is done that allows the person to cast a vote. Arntz said that this process leaves the system open to fraud, TMZ reported.

Last week, Trump said in a tweet: “Of course there is large-scale voter fraud happening on and before election day.”

According to the Guardian, opinion polls suggest that 41 percent of the voters believe in Trump’s charges of a “rigged election.” Another survey revealed that more than two-thirds of all Republicans believe that if Hillary Clinton wins the elections, it will be because of illegal voting or vote-rigging.

"The election is absolutely being rigged by the dishonest and distorted media pushing Crooked Hillary – but also at many polling places – SAD," Trump tweeted Saturday.

Amid the growing debate about voting fraud, Rick Hasen, a professor at the UC Irvine School of Law and the author of "The Voting Wars," said why rigging an election is not that easy.

"[Trump] He's talking about voter impersonation fraud — people going and voting five or 10 or 15 times — and if you've ever been to a polling place, and I believe Donald Trump has been, you know it's impossible to vote five or ten or fifteen times. Once you sign in, that's it," Hasen reportedly explained. "You'd have to vote pretending you’re someone else, or you'd have to register under false names and to do this on a large-enough scale to affect a presidential election, you're talking about tens of thousands of people affecting, potentially, hundreds of thousands of votes. We've just never seen in modern American elections anything along that scale."