Trump, who has frequently repeated unfounded conspiracies of fraud with mail-in votes, has said he'll be ahead in Pennsylvania before the outstanding mail-in votes are counted - and that the only way he'll lose the election is "when the other side cheats."
Biden, who like Trump has made multiple trips to Pennsylvania, said earlier this month that Trump and his Republican allies are trying to "throw into question the legitimacy of the election." The former vice president urged Pennsylvanians to make a plan now to vote.
Pennsylvania is taking steps to scale up for Nov. 3. Counties are investing millions of dollars on new ballot sorters, high-speed scanners and other equipment and staff to handle the projected 3 million mail-in ballots for the Nov. 3 election. Even with those upgrades, many elections officials still think it could take days to count them all if the state legislature doesn't change a law forbidding them to start processing ballots until 7 a.m. on Election Day.
The Pennsylvania Supreme Court also ruled on Sept. 17 that mail-in ballots can be accepted up to three days after the election, and that counties can use drop boxes to collect mail-in ballots. Trump's campaign has sued in federal court to challenge how drop boxes can be used and allow poll watchers in a county even if they don't live there as now required by commonwealth law.
While voting advocates hailed the ruling allowing ballots postmarked before the election to be counted, it could add to the number of uncounted ballots after Election Day if counties can't start processing them early and voters wait until the last minute, said Lisa Schaefer, executive director at County Commissioners Association of Pennsylvania.
"It makes it very likely that many of our counties will not be able to complete it that day," said Schaefer said of the counting of mail-in ballots.
In Philadelphia, the city is using a $10 million grant from the nonprofit Center for Tech and Civic Life to rent 125,000 square-feet of space in its convention center to install new equipment and staging areas with the ability to tabulate about 200,000 mail-in ballots on Election Day, said Commissioner Omar Sabir.
But as many as 350,000 or more ballots are expected, and even with teams working around the clock, it could take 36 hours after the election or longer to finish the counting because envelopes have to be cut open and ballots extracted, flattened and run through scanners - all with the risk COVID-19 could hit workers and disrupt the operation, he said.
"We're not going to have all those mail ballots completed on election night," Sabir said. "It's not going to happen. It's just not."
Bucks County outside of Philadelphia is a battleground where Hillary Clinton topped Trump by only 2,699 votes in 2016 and Barack Obama edged Mitt Romney by 3,942 votes in 2012. The county is adding equipment including a 40-foot ballot sorter it calls "the dragon" but still expects it'll take at least two days after the election to process as many as 250,000 mail-in votes, Commissioner Bob Harvie said.