Election 2020 updates: Biden warns of 'dark winter,' pushes masks in pandemic plan

Election officials in Arizona's Maricopa County told ABC News that ballot counting there could take days and will likely continue into the weekend.

Historic voter turnout in Arizona's most populous county has pushed officials to count more ballots than ever before, officials said.

Some 275,000 absentee ballots and 17,000 provisional ballots have not yet been counted in Maricopa County, along with several thousand early votes that need additional research, according to officials. It was unclear how many ballots would be counted Thursday.

Maricopa County accounts for nearly two-thirds of the state's registered voters. The county seat is Phoenix, the capital of Arizona and the fifth-most populous city in the United States.

With 86% of the expected vote reporting, Biden currently leads in Maricopa County -- 51.4% to Trump's 47.2%.

Biden also holds an edge over Trump statewide, with 1,469,341 votes against Trump's 1,400,951 -- 50.5% to 48.1%. About 86% of Arizona's expected absentee vote and 86% of the state's expected vote overall are in.

-ABC News' Jenna Harrison, Whit Johnson and Matthew Seyler

Biden campaign manager Jen O'Malley Dillon on Thursday called Georgia a "toss-up," and said Trump is in the lead, but Biden is cutting into that lead as more outstanding ballots are counted.

Georgia "is going to be very very close," she said, and "at this stage, leans to the vice president."

O'Malley Dillon said Biden is up narrowly in Nevada and is on track to win the state.

She said the remaining ballots to be counted in Pennsylvania are expected to heavily favor Biden.

O'Malley Dillon called Trump's lawsuits "meritless" and an effort to "distract and delay."

As ballots continued to be counted in several key states Thursday morning, Trump took to Twitter demanding, "STOP THE COUNT!"

Trump tweeted again, saying, "ANY VOTE THAT CAME IN AFTER ELECTION DAY WILL NOT BE COUNTED!" Twitter flagged his second tweet as "disputed and might be misleading."

Trump's tweets come as his own campaign team argues that the ongoing vote count in Arizona will prove to be a win for the president.

The president does not have the power to stop the legal vote counting process. Nevertheless, the Trump campaign has filed lawsuits to halt vote counting in Georgia, Pennsylvania and Michigan.

Shortly before Trump's tweets, Biden took to Twitter himself to say, "Every vote must be counted." Biden's tweet included a video with no audio urging "count every vote."

The percentage of expected vote reporting in Pennsylvania has ticked up from 88% to 89%, while the amount of mail-in ballots counted has remained close to 71%.

Biden has continued to narrow his deficit with Trump from 186,755 votes to 164,414.

There are roughly 775,000 outstanding votes that have yet to be counted in the Keystone State, including 717,412 mail-in ballots, according to the Pennsylvania Department of State.

Of the remaining 775,000 or so votes, Biden would need 469,707 -- 60.6% -- to Trump's 305,293 to win Pennsylvania.

-ABC News' Alex Hosenball, Adam Kelsey, Matthew Mosk and Benjamin Siegel