HARRISBURG, Pa. (AP) — President Donald Trump and former Vice President Joe Biden stumped in Pennsylvania on Monday, the last day before Election Day, with counties reporting they have yet to receive almost 700,000 mail-in and absentee ballots in the premier presidential battleground state.

Mail-in and absentee ballots are the subject of a Republican Party lawsuit before the U.S. Supreme Court, leaving doubt around the ultimate deadline for counties to receive them.

With concern around Postal Service delays and its delivery times as coronavirus infections spread, Pennsylvania officials are urging voters to hand in their mail-in ballots and not to put them in the mail.

“Please do not put ballots in the mail, hand deliver your mail ballot to your county election office, satellite election office or other designated drop box or drop-off location," Pennsylvania's top election official, Secretary of State Kathy Boockvar, a Democrat, said during an online news conference Monday. "Do it today. Do not wait.”

All told, voters in Pennsylvania requested nearly 3.1 million mail-in or absent ballots, or more than 10 times the amount received in 2016′s election. They had returned slightly over 2.4 million ballots as of Monday morning, according to state data.

The majority of mail-in ballots are being cast by Democrats, according to state data. Democrats requested more than 1.9 million ballots by mail, and had returned 1.6 million, according to state data. That means Democrats had yet to return more than 300,000 ballots, according to the latest state data.

With so many voters casting ballots by mail for the first time, Democrats in particular have worried that ballot irregularities — like forgetting to sign it or not putting it inside a second “secrecy” envelope — will result in tens of thousands of votes being disqualified.

In Bucks County, a line of dozens of voters — apparently notified by the Democratic Party that there was a problem with their ballot — went out the door at the county government offices on a cold, blustery day Monday.

Susan Fetterman, 57, an attorney from Yardley, forgot to date her ballot. She opted to wait in line to correct her first ballot instead of getting a provisional ballot, because she didn't want to chance the latter being rejected for some other reason, she said.

Her reaction to finding out from her state representative's office over the weekend that her mailed ballot was incorrect?

"Super annoyed,” she said. “I was just like, ‘Come on, me of all people?’ I’m an attorney and should know how to fill things out, right? I wasn’t mad or anything. It’s better to be careful.”

She said she wasn’t worried that it might not be counted.

“I trust the process. I’m not one of these people who thinks there’s a lot of voter fraud," she said. "I do think that there are going to be people to do anything they can to make sure certain votes don’t count.”

Biden was at the other end of the state for three events: kicking off a canvass in Beaver County with labor unions and attending one drive-in rally in Pittsburgh with African American community leaders and another in the evening with Lady Gaga in the city.

At Beaver County Community College, Biden continued his pitch to union and working class workers, promising to “be the most pro-union president you’ve ever seen.”

Trump headlined an early afternoon rally at Wilkes-Barre/Scranton International Airport in northeastern Pennsylvania, near Biden's birthplace and an area of the state where Trump performed well in 2016 in a pair of Democratic-leaning counties. There, he repeated his baseless attacks that the election in Pennsylvania would be marred by fraud designed to beat him, saying that his pollster told him he needs to allow for a 5% swing because of fraud.

“Of course, everyone knows it’s true,” Trump told the crowd.

Trump also complained about a state court order to extend the deadline to receive and count ballots that arrive in the mail to as late as 5 p.m. Friday.

That deadline is in doubt, since the state Republican Party is challenging that order before the U.S. Supreme Court. It is asking the court to reset the deadline to the one in state law, which is 8 p.m. Tuesday, when polls close, and it is not clear if, or when, the U.S. Supreme Court may take up the case.

Late-arriving ballots could take on enormous importance if Pennsylvania turns out to be the crucial state in the election and they are potentially decisive.

The deadline to hand in ballots is when polls close at 8 p.m. Tuesday.

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Catalini reported from Doylestown, Pennsylvania.