Four key battleground states -- Pennsylvania, Wisconsin, Michigan and Georgia -- began Wednesday with tens of thousands of absentee ballots uncounted, leaving the White House race between President Donald Trump and former Vice President Joe Biden up in the air.

Election officials in some states called it a night and planned to resume the count in the morning, while some counties in Pennsylvania weren't even to start tabulating their mail-in votes until later Wednesday morning. The mail-in ballots, which smashed records this year as a result of the Covid-19 pandemic, are expected to favor Biden, whose campaign encouraged Democrats to vote early, while in-person votes on Election Day may have given Trump an advantage.

Trump and his allies have repeatedly called for results to be tallied quickly so that a winner could be declared on election night, though officials technically have days or weeks to complete official counts before state totals are certified. But in three key states -- Michigan, Pennsylvania and Wisconsin -- election officials were not allowed to begin processing absentee ballots until on or just before Election Day, after Republican-led state legislatures successfully opposed changing laws to allow earlier preparations like other states.

As Biden gave remarks early Wednesday calling for patience while workers continued to count, Trump falsely tweeted that Democrats were "trying to STEAL the Election," even though the counting delay was fully expected. Twitter quickly labeled the tweet, saying it was "disputed and might be misleading."

In Pennsylvania, where officials couldn't begin processing hundreds of thousands of early ballots until Tuesday, counties made their own decisions about how to prioritize the crush.

In Philadelphia, around 75,000 mail-in ballots had been counted out of 350,000 that had been returned by late Tuesday, with another chunk expected to be counted before officials stop for the night, said City Commissioner Al Schmidt. Election workers in Luzerne County, a northeastern county near Scranton, stopped counting mail-in ballots on Tuesday evening and will resume Wednesday morning, according to county manager David Pedri. He said the county had counted about 26,000 mail-in ballots of roughly 60,000 cast.

Montgomery County, northwest of Philadelphia, planned to count "24 hours a day until completion," according to county spokesperson Kelly Cofrancisco.

In Georgia, where rules allowed for pre-processing, major counties nevertheless reported backups and sent workers home rather than finish counting overnight.

By 10:30 p.m. ET Tuesday, Fulton -- which is the state's largest county and includes Atlanta -- had counted all in-person votes and stopped counting mail-in ballots for the evening. Officials there plan to resume counting the absentee ballots Wednesday morning at 8, Fulton County spokeswoman Jessica Corbitt told CNN late Tuesday.

As of Tuesday evening, 86,000 absentee ballots had been accounted for, according to Corbitt. According to data from Georgia Secretary of State Brad Raffensperger, Fulton County had roughly 134,000 outstanding absentee by mail ballots going into Election Day, leaving at least 48,000 to be counted.

Georgia ran into other issues too. A pipe burst early Tuesday morning at Atlanta's State Farm Arena above the processing room for all absentee ballots in Fulton County, delaying counting there, said county spokeswoman Regina Waller. No ballots were damaged, according to Waller.

A suspected problem with voting tabulation software has caused delays in the counting of as many as 80,000 mail-in ballots in Gwinnett County, which is east of Atlanta, according to a county spokesman. Officials believe the software erroneously identified flaws in the way voters filled out the ballots.

Legal fights loom

Mail-in ballots could also be the source of post-election legal fights. Republicans filed a lawsuit challenging at least 1,200 absentee ballots in Democratic-leaning Montgomery County outside of Philadelphia. A federal judge will hear the challenge on Wednesday morning.

Three Republicans observing the processing of mail-in votes described to a federal court how they saw absentee ballots with possible technical issues and believed elections officials might impermissibly try to give voters opportunities to fix ballots with issues that would have caused them to be thrown out. The Republicans alleged that the county had begun processing mail-in ballots too early and was illegally trying to allow voters to fix defects, such as by adding missing inner envelopes.

Also in Pennsylvania, GOP Rep. Mike Kelly and others filed a lawsuit in state court Tuesday evening accusing the Pennsylvania secretary of state of illegally advising that provisional ballots could be offered to absentee voters whose ballots would be rejected.

Officials in the states where ballots were still outstanding urged patience while the results are calculated. Pennsylvania Gov. Tom Wolf, a Democrat, released a video Tuesday evening cautioning voters to "remain calm" while the vote count is ongoing.

"Across the state, dedicated county workers are ready to tirelessly make sure everyone's vote counts," Wolf said.

In Milwaukee, absentee ballots were being counted at a central facility, and the county's elections director, Julietta Henry, predicted they would be done by about 5 a.m. Wednesday, according to a spokesperson. "It is in no way, shape or form unusual for jurisdictions to still be counting into the morning," said Meagan Wolfe, the chief elections official for the state of Wisconsin.

Michigan Secretary of State Jocelyn Benson, who had suggested in the weeks before the election that counting could take until Friday, predicted to reporters Tuesday evening that the state could "potentially see a full result of every tabulation out of Michigan in the next 24 hours." The pace in Detroit, where 120,000 absentee ballots had already been counted by late Tuesday, underscored that the state is on track to be done counting results sooner than expected, Benson said.

Detroit Deputy Clerk Andre Gilbert told CNN that the city, the largest in Wayne County, will have unofficial results "probably early morning," with day-of votes finishing up soon and absentee ballots taking longer. Based on the last official report from the city clerk, approximately 92,000 absentee ballots still remain to be counted.

Technology issues in multiple states

Several states had other issues pop up that led to delays in counting ballots. In Outagamie County, Wisconsin, which is outside Green Bay, poll workers on Tuesday were working to transfer votes from around 13,500 misprinted absentee ballots to clean ballots that won't jam the electronic tabulating machine, the county clerk told CNN.

In South Carolina, a printing error delayed the counting of 14,600 absentee-by-mail ballots in Dorchester County, north of Charleston, until later in the week, state elections officials said. The marks at the tops of the ballots that alert the scanner to start tabulating votes are too small for the scanner to read, said Todd Billman, executive director of Dorchester County Elections.

Election officials sent out a statement Tuesday evening saying that they plan to run the ballots through a scanner again starting Wednesday morning. If that doesn't work, Billman told CNN, each vote may have to be manually duplicated by a poll worker using a touch-screen voting machine, with a witness observing the process.

An internet outage occurred Tuesday in Osceola County in central Florida, and ballots were taken to the county's elections office for counting, said Brandon Arrington, a county commissioner. Arrington said he was not sure how much of a delay this would cause or how many ballots were affected. Osceola includes the town of Kissimmee, just south of Orlando.

While election officials expressed concern about the challenges of voting during a pandemic, the battleground states reported that voting at polling places was mostly smooth, with only isolated incidents. Michigan Secretary of State Benson said Tuesday that "precincts are islands of calm," while the spokesman for Florida's Broward County Supervisor of Elections said the day was "boring."

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