The Keystone State is a big prize for Trump, because even if he wins Florida and holds battlegrounds he won in the South and Southwest, he would still be short of 270 electoral votes without it.
Biden, meanwhile, has several paths to victory without winning Pennsylvania.
As of Wednesday afternoon, ballots were still being counted across the Keystone State.
The Trump campaign announced that it was suing to stop the vote count, charging "Democrats are scheming to disenfranchise and dilute Republican votes."
In a statement, Trump 2020 Deputy Campaign Manager Justin Clark wrote, "We are suing to stop Democrat election officials from hiding the ballot counting and processing from our Republican poll observers-observers whose only job is to make sure every valid ballot is counted, and counted once. The eyes of the country are on Pennsylvania, but Pennsylvania has kept eyes off of the absentee ballot counting process all along, and that must stop today. In Philadelphia and elsewhere, Democrat officials forced our observers to stay 25 feet or more from the counting process, leaving no meaningful way whatsoever for our observers to do their jobs. We are also suing to temporarily halt counting until there is meaningful transparency and Republicans can ensure all counting is done above board and by the law."
The campaign is also suing Pennsylvania for allegedly changing state election code regarding first-time voters providing identification in order to vote.
Pennsylvania's three largest counties - Allegheny, Montgomery, and Philadelphia - which sent out the most absentee ballots to voters, will continue counting their mail-in ballots through the night, and into Wednesday.
State Secretary Kathy Boockvar said Wednesday all Pennsylvanians' votes will be counted, including military ballots.
"And again, I'll also just remind everyone, military and overseas ballots are not due until a week after Election Day. So next Tuesday is the deadline for military and overseas voters to cast their ballots. And we want to make sure that not only every civilian absentee and mail-in valid voter is counted, but also that every man and woman who are serving our country, that their votes are counted," Boockvar.
Philadelphia Continues Counting
Philadelphia City Commissioners, elected officials who oversee elections, announced Wednesday morning that ballots will continue to be counted after experiencing a delay with counting in-person votes.
Election officials said as of about 4 p.m., they've counted more than 230,000 absentee mail-in ballots and still have more to go. They aren't saying when exactly they expect to be done, but said sometime Thursday morning is a good target.
Republican Philadelphia City Commissioner Al Schmidt tweeted Wednesday morning that, "Philadelphia will NOT stop counting ALL legitimate votes cast by eligible voters. And we will report and report and report until the last vote is counted."
Two hours later, he said the city's results page was updated to "include more than 65,000 mail-in votes cast by Philadelphia voters."
Moments ago, we updated our results page to include more than 65,000 mail-in votes cast by Philadelphia voters, with many more yet to count. You can follow our results page here: https://t.co/Acrk5q3sYy— Commissioner Al Schmidt (@Commish_Schmidt) November 4, 2020
Election officials are describing it as an army of people working behind the scenes overnight and into the morning to get all votes counted.
Commissioners said there was a delay getting in-person voting machines back to regional centers to finish counting votes after polls closed.
Democratic City Commissioner Lisa Deeley said they must get those remaining votes into the counting machines before adding additional mail-in ballots because of how the system is configured.
They hope to get back on track while managing the record number of more than 400,000 mail-in ballots returned in Philadelphia.
"We've never had to count this large number of mail-in ballots and have an in-person election. We thought we had timed it right, but truthfully we've never done it before in this large number," Deeley said.
Officials in Pennsylvania had to wait until polls open on Election Day to begin counting absentee mail-in ballots because Democratic Gov. Tom Wolf and Republican leaders of the state legislature could not agree on a plan to allow ballot processing before Election Day.
President Trump spoke after polls closed and pledged to go to the U.S. Supreme Court to stop any further counting of ballots across the country.
City Commissioners said the next update is planned to happen around 9 a.m. and that they expect to have large number of results.
Allegheny County has scanned and uploaded 151,022 mail and absentee ballots, less than half of the 340,000 mail in ballots received as of this early this morning. In addition to finishing that process over the next few days, they will also have to process any ballots they receive between now and Friday. While the in-person vote could be fully uploaded by Wednesday, it's likely that it will take several additional days for a full count of absentee mail-in ballots.
Elections in Pennsylvania
*Counties are colored red or blue when the % expected vote reporting reaches a set threshold. This threshold varies by state and is based on patterns of past vote reporting and expectations about how the vote will report this year.
Local 2020 Election Results
While campaigning in Pennsylvania on Monday, the two men broke sharply on the voting process itself.
The president threatened legal action to stop counting beyond Election Day. If Pennsylvania ballot counting takes several days, as is allowed, Trump charged that "cheating can happen like you have never seen."
Biden, in Pittsburgh, pushed a voting rights message to a mostly Black audience, declaring that Trump believes "only wealthy folks should vote" and describing COVID-19 as a "mass casualty event for Black Americans."
"We're done with the chaos, we're done with the tweets, the anger, the hate, the failure, the irresponsibility," said Biden, whose campaign has focused on increasing turnout for Black voters, who could prove the difference in several battleground states.
All of Pennsylvania's 18 members of Congress sought reelection, and as of 1 a.m. Wednesday, at least 11 won - Republicans John Joyce, Fred Keller, Guy Reschenthaler, Glenn Thompson, Lloyd Smucker and Dan Meuser; and Democrats Mike Doyle, Dwight Evans, Brendan Boyle, Mary Gay Scanlon and Madeleine Dean.
The race for Pennsylvania's Attorney General is very close.
With 66% of the precints reporting, Republican Heather Heidelbaugh is leading incumbent Democrat Josh Shapiro by 9 points.
Republicans and a voter outside Philadelphia filed a federal lawsuit accusing Montgomery County officials of illegally processing mail-in ballots before Tuesday for the purpose of allowing voters to fix problems with their ballots.
A federal judge in Philadelphia set a hearing for Wednesday morning on the Republican bid to stop the count of 49 ballots that were amended and returned in the suburban county.
The state's high court has not prohibited counties from allowing voters to fix their ballots, said Kelly Cofrancisco, a county spokesperson.
And in a lawsuit filed Tuesday night in a statewide appellate court, Republican U.S. Rep. Mike Kelly of Pennsylvania and five other plaintiffs asked to block counties from allowing voters whose mail-in ballots were disqualified to be able to cast a vote by provisional ballot.
The lawsuit said the state Supreme Court has already ruled that state law provides no such avenue for a voter to fix a disqualified vote. In Oct. 21 guidance to counties, state elections officials said a voter whose mail-in or absentee ballot was rejected could still vote in person by a provisional ballot.
Boockvar insisted that the practice singled out by the lawsuit is legal. Regardless, she said there aren't "overwhelming" numbers of voters who cast a provisional ballot after their mail-in ballot was disqualified, but she did not give an exact figure.
The state Supreme Court - citing Postal Service delays, the huge number of people voting by mail because of COVID-19, and the strain on county boards of election - ordered counties to count mail-in ballots received as many as three days after the vote, so long as they were mailed by Election Day.
The U.S. Supreme Court rejected a Republican effort to block the counting of late-arriving mail-in votes, but it could revisit the issue.
Control of the state House was also at stake, with Democrats needing nine seats to seize the majority from Republicans after a decade out of power. Democrats also had a gap to make up in the state Senate.
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1st Congressional District: Brian Fitzpatrick, Christina Finello
In the Bucks County-based 1st District in suburban Philadelphia, Rep. Brian Fitzpatrick is being challenged by Democrat Christina Finello. She maintains that Fitzpatrick hasn't stood up to President Donald Trump, while Fitzpatrick says he's independent of Trump.
Finello was a top official in Bucks County's Division of Housing and Human Services before she ran for Congress.
Fitzpatrick, a former FBI agent who succeeded his brother in the seat, is one of just three House Republicans in the entire country running for reelection in a district won by Democrat Hillary Clinton in 2016's presidential contest.
Fitzpatrick won 2018's election by 2.5 percentage points in 2018, when he was outspent nearly four-to-one by his wealthy Democratic rival and millions flowed in from outside groups.
Fitzpatrick voted with every other Republican against a $2.2 trillion relief bill that passed the Democratic-controlled House on Oct. 1, and has stalled in the Republican-controlled Senate.
As of Wednesday afternoon, ABC News projects Fitzpatrick will win.
6th Congressional District: Chrissy Houlahan, John Emmons
As of Wednesday afternoon, ABC projects Democratic incumbent Chrissy Houlahan will win the race against Republican John Emmons.
The district includes almost all of Chester County, and parts of Berks County.
7th Congressional District: Lisa Scheller, Susan Wild
As of 12 a.m. Thursday, with 85% of precincts reporting, Republican Lisa Scheller is leading Democratic incumbent Susan Wild 51% to 49% in Pennsylvania's 7th congressional district, in Lehigh and Northampton Counties as well as parts of Monroe County.
17th Congressional District: Conor Lamb, Sean Parnell
A Biden backer, U.S. Rep. Conor Lamb, a Pittsburgh-area Democrat who has pushed back against the party's left wing on energy, recalled hearing concerns about Biden's position on fracking during events at union halls and elsewhere.
But, Lamb said he is satisfied with Biden's position on fracking and his commitment to blue-collar labor union jobs after three decades of Democrats losing clout among working-class voters in western Pennsylvania.
"He's picking up ground," Lamb said. "We'll see how much he's picked up, but I'm definitely enthusiastic that he's out there fighting for it."
Lamb is running against Sean Parnell, an Army veteran and Purple Heart recipient who served on the Afghan-Pakistan border. Parnell spoke at the Republican National Convention.
As of Wednesday night, Lamb had 188,523 - 49% of the votes, and Parnell had 195,161 - 51% of the votes, according to the Associated Press.
10th Congressional District: Scott Perry, Eugene DePasquale
In Pennsylvania's 10th District, Democrats have long been enthused about their recruit, state Auditor Eugene DePasquale. He's trying to unseat GOP Rep. Scott Perry, a member of the House Freedom Caucus.
As of Wednesday night, DePasquale had 160,818 - 45% of the votes and Perry had 195,717 - 55 % of the votes, according to the Associated Press.
RELATED: Justices deny fast, new look at Pennsylvania ballot deadline
In a persistent gambit to win the battleground state of Pennsylvania, Trump has worked to peel off voters connected to its booming natural gas industry and accuse former Vice President Joe Biden of plotting to shut it down.
Biden calls that attack a flat-out lie. It's not clear how many votes are being decided by Trump's fracking claims in a contest where the vast majority of voters had already made up their minds.
The gas industry has flushed money into some local economies, but it has inspired a backlash in other communities, most notably in Philadelphia's suburbs, and, for many voters, it simply doesn't rank as deciding factor in the race.
President Donald Trump makes campaign stop in Lehigh Valley, Pennsylvania