The Keystone State is a must-win for the president, because its 20 Electoral College votes would put his opponent over the 270-vote victory line.
However, his opponent now has a lead of almost 29,000 votes following the return of mail-in ballots. There are tens of thousands of votes left to count, and many of those are thought to favour Mr Biden.
While a close margin and a large number of votes left to count make it too early to call, we may know who will win the state at the weekend. If Mr Biden wins, then he wins the presidency.
Nonetheless, a potential recount – if the final tally is close enough – could potentially push a definitive result back several days.
The secretary of state, Kathy Boockvar, said on Thursday evening that election officials were on the “home stretch” in terms of counting. More than 99 per cent of votes have now been counted.
“The overwhelming majority of ballots will be counted by Friday – we actually have counted the overwhelming majority of ballots, but because it’s a close race, it’s not quite clear yet,” Ms Boockvar said.
Mr Trump, who held a 675,000-vote lead early on Wednesday, prematurely declared victory and claimed that “we're winning Pennsylvania by a tremendous amount ... It's not like, 'Oh, it's close'”.
However, by late Thursday night his advantage had slipped to about 24,000 votes thanks to mail-in ballots falling overwhelmingly in Mr Biden's favour. The situation has now flipped with the Democrat leading.
Under state law, elections officials are not allowed to process mail-in ballots until election day, in contrast to other states such as Texas.
If there is less than half a percentage point’s difference between the candidates, state law dictates that a recount must be held.
In 2016’s election, before coronavirus and the advent of mass mail-in voting, Mr Trump’s victory over Hillary Clinton in Pennsylvania was recorded by the Associated Press at 1.35am on 9 November, the morning after election day.
Meanwhile, Philadelphia police said they arrested two men on Thursday for not having permits to carry the guns they were armed with near the state convention centre, where votes were being counted.
Police said they received information earlier in the day that individuals armed with firearms were on their way to the convention center in Philadelphia in a Hummer truck.
The two men arrested acknowledged that the Hummer spotted by officers near the centre was their vehicle, police said. An additional firearm was recovered from inside the vehicle.
Both men will face firearm charges but have not been formally charged yet.
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