Editor's note: This story has been updated to reflect the number of potential votes that could be impacted in 2020 based on the 1.4 million bail-in ballots cast in 2016.


Five to 10 percent of mail-in ballots — potentially thousands of ballots cast in the battleground state of Pennsylvania — could be rejected on Nov. 3, U.S. election officials said Wednesday.


Members of the U.S. Election Assistance Commission, a federal agency formed in 2002, strongly encouraged Pennsylvanians to vote in person on Election Day.


EAC members predicted a flurry of legal battles over the handling of mail-in ballots, the signature matching techniques used by local elections officials, and the stamps placed on official election mail as it's received by local mail distribution centers.


"In many places, we’re seeing rejection rates somewhere in the range of between five and up to 20%, and that should not be acceptable to anyone," said Christy McCormick, a U.S. Election Assistance commissioner nominated by President Obama in 2014.


It’s a "very reasonable prediction" that 5% of mail-in ballots could be rejected in Pennsylvania, said Donald Palmer, who was nominated by President Trump and confirmed  by the U.S. Senate in 2019.


A federal lawsuit filed by the Trump campaign on June 29 aims to curb mail-in voting.


More people voted by mail than voted in person during the June 2 Pennsylvania primary, voter records show.


Of the 1.4 million mail-in ballots, 40% came from Bucks, Montgomery and Philadelphia counties.


More: How this state could swing the presidential election to Trump again


More: Voting by mail in Pennsylvania? Here's what you need to know


In some states, voters are familiar with the mailing process.


Colorado, Hawaii, Oregon, Washington and Utah conduct all votes by mail. California, Nebraska and North Dakota allow counties to decide whether residents vote by mail.


In states where voters are unfamiliar with the process, the ballot rejection rate can vary from  1 percent to 10 percent, said McCormick. She described any number of issues that can lead to rejection of a mail-in ballot.


"When you see some paper ballots and the way some people mark up their paper ballots, it's alarming," McCormick said. "We see circles, arrows, people writing things in, and double voting."


"Voters need to read the instructions carefully," she continued. "Make sure that you don't put any extraneous marks on your ballot. It's important that you sign the ballot where it needs to be signed. It's important that you put your ballot into the secrecy envelope."


The U.S. Election Assistance Commission was formed after the 2000 presidential contest between Democrat Al Gore and Republican George W. Bush.


A legal battle ensued after a recount in Florida, and the case ultimately was decided by the U.S. Supreme Court.


"If you go back to 2000, we had voting equipment with a margin of error that was just unacceptable to the American people," said Palmer. "We worked to increase the accuracy of the voting systems in the polling places so it was more forgiving of voters (who might make a mistake).


"We want voters to understand that you could have a more fatal error if you vote by mail then if you were to vote in person," he continued. "No one anticipated a situation where 60 percent (of the people) might be voting by mail. I'm concerned about the impact to voter confidence."


This article originally appeared on Bucks County Courier Times: 5 percent of Pennsylvania mail-in ballots could be rejected