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

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!”

Note: Updated to correct outstanding number of ballots

About 190,150 ballots in Nevada have yet to be counted. Of those, 90% are in Clark County, home to Las Vegas, state officials said.

Of the 190,000 ballots statewide, 123,554 are ballots that were either returned by mail or dropped of in person at a ballot drop-off location, and 66,596 are ballots that were cast in person at a polling place, either during early voting or on Election Day, by voters who took advantage of Nevada’s new same-day voter registration law.

Mail-in ballots will continue to be counted through Nov. 10 so long as they arrive postmarked by Nov. 3. "We cannot estimate with any degree of accuracy how many ballots might fall into this category," Nevada Secretary of State Barbara Cegavske said.

-ABC News' Kendall Karson and Cheyenne Haslett

Biden addressed the nation Thursday afternoon, saying, "Each ballot must be counted."

"Democracy is sometimes messy. It sometimes requires a little patience," Biden said.

"We have no doubt that when the count is finished, Senator [Kamala] Harris and I will be declared the winners," he said. "So I ask everyone to stay calm... The process is working. The count is being completed and we'll know very soon."

Before speaking about the election, Biden mentioned COVID-19, saying he and Harris just finished briefings on the pandemic and the economy.

"Cases are on the rise nationwide and we're nearing 240,000 deaths," Biden said. "Our hearts go out to each and every family that has lost a loved one to this terrible disease."