Philadelphia Mayor Jim Kenney, a Democrat, warned residents in his city that counting the mail-in ballots “will easily take several days” after Election Day, which increases the possibility that it could be a while before the election is settled.

Pennsylvania has been seen as one of the most pivotal states in the election. President Trump hopes to attract a massive surge in voters from outside Philadelphia and its suburbs in order to offset the highly Democrat city. Kenney’s announcement could open the possibility that Trump opens up a massive lead going into Nov. 4, but could see that advantage evaporate as the vote from the city gets tallied.

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Kenney’s letter was also signed by Lisa Deeley, the chairwoman of the Philadelphia City Commissioners, and it assured voters in the city that the “process and procedures favor no party or group.” They said that staffers are not allowed to start counting the mail-in ballots until Election Day.

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Nate Silver, the pollster, told ABC’s “This Week” that he thinks the 2020 election will come down to  Pennsylvania.

“Pennsylvania has not bumped up to a seven or eight-point Biden lead, like we see in Michigan and Wisconsin,” he said. Biden leads Trump by 4.9% in Pennsylvania, according to FiveThirtyEight’s updating average of 2020 presidential general election polls, compared to a more sizeable lead of 8.4% in Michigan and 8.6% in Wisconsin.

Bloomberg reported that Philadelphia has already received 400,000 mail-in ballots and pointed out that three-quarters of the voters there are registered Democrats. They noted that the city could very well determine who wins the state.

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A higher percentage of Pennsylvanians who requested mail-in ballots are Democrats and there is the potential for a “red mirage,” which describes a situation where Republican candidates could appear to have an outsized amount of support as votes are reported on Election Day – followed by a shift toward Democratic candidates in the days that follow.

Joe Biden was in Philadelphia on Sunday, the largest city in what is emerging as the most hotly contested battleground in the closing 48 hours of the campaign. He participated in a “souls to the polls” event that is part of a nationwide effort to organize Black churchgoers to vote.

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“I think it’s absolutely true that more Republicans will vote in-person,” Republican Bucks County Commissioner Gene DiGirolamo told Fox News. “If you’re just announcing the results from people who voted at the polls, I think that number is going to show … [more] favor for the president than it will for Biden.”

Fox News' Brittany De Lea, Talia Kaplan and the Associated Press contributed to this report