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

Pennsylvania Secretary of State Kathy Boockvar, providing an election update Thursday evening, told reporters they're coming in the "homestretch" but that ballot counting will continue into the evening.

"Hundreds of thousands of ballots have been counted so far today, and we're in very good shape, but there's still some to count," she said. "Stay tuned."

Boockvar said the majority of in-person votes have been counted but they're still in the process of counting mail-in and absentee ballots, as Pennsylvania is among the handful of states that did not begin processing absentee ballots until Election Day morning. She expected an "overwhelming majority" will be counted by Friday. 

"But then there's still going to be provisional ballots, the military and overseas ballots and so forth. So the closer the race is, the longer it takes," she said, adding "it's not quite clear yet who the winner is."

If Biden were to win Pennsylvania's 20 electoral votes, it would put him over the top of the necessary 270 votes to win the presidency.

Trump will speak from the White House at 6:30 p.m. ET Thursday. The president last spoke to the public in the early hours of Wednesday.

Pennsylvania Democrats and Secretary of State Kathy Boockvar have asked the U.S. Supreme Court to deny the president's request to formally join a pending challenge to the state's three-day mail ballot deadline extension. 

In a pair of filings, Boockvar argued that allowing Trump to join "would significantly prejudice the existing parties" and that he has not provided any justification for why he should be added to the case at this stage.

The case has been ongoing for six weeks. 

The Pennsylvania Supreme Court previously denied Trump's motion to join the lawsuit citing "insufficient" justification of mere "generalized grievance." 

Pennsylvania's Democratic Party also urged the court to reject the president's assertion that the case could determine who wins the presidency. 

"You may not need to hear and decide these cases," they wrote, noting that the universe of contested votes is not yet fully known nor is the math as to whether it would have any impact on the outcome. 

-ABC News Senior Washington Reporter Devin Dwyer

The Trump campaign held a news conference in Phoenix Thursday afternoon and said it won’t be announcing any legal challenges in Arizona yet as the count proceeds in the state but that “all options are on the table.”

"All of the litigation strategies are being thought about and contemplated. There's been no decisions on anything as of yet, but absolutely all options are on the table,” said campaign adviser David Bossie.

The campaign projected confidence that Trump will ultimately overcome Biden, who is currently leading in the state by nearly 70,000 votes. 

Trump supporters interrupted the press conference with chants of "Count our votes!" after the president tweeted this morning, “STOP THE COUNT!”