Ramping up his rhetoric ahead of the 8 November poll, Trump said last night at a rally in the state: "The only way we can lose, in my opinion - and I really mean this, Pennsylvania - is if cheating goes on. I really believe it."
Trump's attack focused on the fact that the northeastern state, which has 20 of the total 538 electoral college votes, does not require voters to show a form of identification when they turn up to cast their ballot. The outspoken billionaire said this raised fears people could vote on more than one occasion.
Trump said: "We're gonna watch Pennsylvania. Go down to certain areas and watch and study and make sure other people don't come in and vote five times."
Read more: Clinton hot favourite to win 2016 election
Pennsylvania has not voted for a Republican candidate in any of the the last six presidential elections, with George H Bush the last successful contender in 1988.
Polling by American statistician Nate Silver, who shot to fame by predicting 49 of the 50 states in the 2008 election, puts Trump's chances of winning the state at just 12 per cent, expecting Democrat Hillary Clinton to come out on top by a margin of around eight points.
Despite the odds being against him, Trump said: "The only way they can beat it, in my opinion - and I mean this 100 per cent - is if in certain sections of the state they cheat, OK? So I hope you people can sort of not just vote on 8 [November], go around and look and watch other polling places and make sure that it's 100 per cent fine."
US 2016 Presidential Election: Battleground states
|Georgia||Clinton by 0.5 points|
|Nebraska (Second district)||Clinton by 0.5 points|
|Arizona||Clinton by 0.6 points|
|South Carolina||Trump by 1.8 points|
|Missouri||Trump by 2.6 points|
|North Carolina||Clinton by 4.5 points|
The comments fit into Trump's tactic of questioning Clinton's integrity and trust - perceived to be one of the Democratic candidate's chief weaknesses. He routinely refers to the former Secretary of State as "crooked Hillary".
The fears about voter fraud in Pennsylvania also tap into a fringe Republican conspiracy theory regarding the fact that in the 2012 election there were 59 districts in the state, mostly impoverished African American communities in Philadelphia, where not a single vote was cast in favour of Republican candidate Mitt Romney.