The Supreme Court Monday night -- in a closely watched election case -- rejected a request by Pennsylvania Republicans to block a state court order that now allows mail ballots arriving up to three days after Nov. 3 to be counted.

Chief Justice John Roberts appears to have joined the three liberal justices in agreeing to reject the GOP request. Justices Clarence Thomas, Samuel Alito, Neil Gorsuch and Brett Kavanaugh all said they would have granted an emergency stay of the state court order.

The split decision by the court is a major victory for Democrats who had argued that the state’s statutory requirement that mail-in ballots must be received by Election Day violates the state constitution. They said the pandemic, coupled with documented postal service delays, would have led to potential widespread disenfranchisement.

Republicans, who have been defending state law and wanted to preserve the Election Day deadline, argued the extension was an “open invitation to cast ballots after Election Day” and create “chaos” and fraud.

Legal experts have considered this case a major bellwether for a raft of other ballot related cases percolating on the court’s docket.

Nancy Patton Mills, chairwoman of the Pennsylvania Democratic Party, said it was a "significant victory" for voters and that the "Supreme Court was right to throw out (Harrisburg Republicans) latest bad faith effort to muddy this election."

However, Pennsylvania GOP Chairman Lawrence Tabas said Republicans disagree with the decision and said the fact that it was a 4-4 decision "only underscores the importance of having a full Supreme Court as soon as possible."

“To be clear, the Supreme Court decided not to grant a stay -- which does not mean the actions of the Pennsylvania Supreme Court would withstand a legal challenge to their Judicial overreach should the Court hear the case,” Tabas continued.

Also weighing in were parties on either side of the Supreme Court confirmation fight.

"It is disturbing that 4 justices would have granted the stay sought by the Republican party. This underscores why Judge Amy Coney Barrett should not be confirmed to the Supreme Court at this moment because there is a good chance that she would be casting the deciding vote on matters related to the 2020 election," Kristen Clarke, president and executive director at the Lawyer's Committee for Civil Rights Under Law said in a statement.

The Honest Elections Project’s executive director, Jason Snead, cited in a statement other cases moving through the courts and said, "It will certainly make headlines, but it is neither a license for other courts to modify election deadlines, nor a resolution of this crucial issue. There are clearly sufficient votes on the Court to hear this case should a cert petition be filed."

-ABC News Senior Washington Reporter Devin Dwyer and Benjamin Siegel

During the two minutes that a candidate has to answer a question, the other candidate's microphone will be turned off. This is designed to prevent the kind of serial interruptions seen from the president in Cleveland at the first debate.

During the follow-up discussions, however, both mics will be on and there will be no mute button. The moderator will attempt, as always, to keep equal time.

The Trump campaign responded to the announcement in a statement that said the president "is committed to debating Joe Biden regardless of last-minute rule changes," but continued to attack the commission for "their latest attempt to provide advantage to their favored candidate."

A Trump campaign official told ABC News that they first learned of the change through media reports.

-ABC News Chief White House Correspondent Jonathan Karl and Will Steakin

As concerns about Trump's campaign cash crunch mount with just two weeks to go until Election Day, the campaign announced a $55 million ad blitz in several battleground states for the two final weeks of the presidential contest.

The ads, funded by the campaign and the Republican National Committee, will target a string of key states along the Sun Belt and the Rust Belt, including Arizona, Iowa, Michigan, North Carolina, Florida, Pennsylvania, Georgia, Nevada and Wisconsin, as well as in Iowa and Ohio, campaign manager Bill Stepien said on a call with reporters on Monday.

The new ad spending comes on the heels of a growing uneasiness within the president's reelection campaign as it continues to implement belt-tightening measures including cutting back on television ad spending in the final stretch of the campaign.

The new ads will zero in on appealing to older voters, RNC Chairwoman Ronna McDaniel said on the call, including one ad that would focus on "Medicare savings" achieved under the Trump administration, which she described as "truly phenomenal."

Recent polls have shown older voters moving toward Biden.

The campaign described the new ad blitz as a 40% increase in Trump's team's ad spending, though it's not yet clear what the new spending will bring the total amount for the final two weeks to. It's also not clear how much of the new spending will be allocated to each state.

Ad research firm Kantar/CMAG was still aggregating new ad placements that were coming in throughout the day on Monday, but earlier, CMAG's analyst Mitchell West told ABC News that the firm's researchers had spotted some cancelations from Minnesota, Wisconsin and Ohio and some additional placements in Florida, Arizona and Georgia.

As of Monday morning -- without the new spending included -- the Trump campaign had a total of $45 million worth of airtime reserved for the two final weeks, including $12 million in Florida, $5.6 million in North Carolina, $5.3 million in Ohio and $4.3 million in Minnesota, according to CMAG's data.

-ABC News' Will Steakin and Soorin Kim

At a campaign rally in Prescott, Arizona, Trump fired off attacks on Biden, Harris, the Obamas, the Cuomos and Democratic Senate candidate Mark Kelly, among others, and continued his public critique of the nation’s top infectious disease expert Dr. Anthony Fauci.

"Biden wants to lock it down. He wants to listen to Dr. Fauci," Trump said, insisting people are "getting tired of the pandemic." He repeated his earlier criticism that Fauci made bad calls about wearing masks and shutting off travel from China, adding that Fauci is a "wonderful guy" but "he just happens to have a very bad arm."

Moments before Trump took the stage, he slammed the task force doctor on Twitter, sarcastically comparing his TV stardom to the legendary Bob Hope and mocking Fauci's Washington Nationals face mask because of his famously bad first pitch at Nationals Park.

Taking questions from reporters ahead of a rally in Prescott, Arizona, Trump referred to his exchanges with the press as “debate prep” and did not indicate he was doing any other preparation -- while Biden has hunkered down to get ready for their final matchup on Thursday.

"What am I doing to prepare? I'm doing this," Trump said. "I've done -- I’ve done very well in debates and you know, you do what you do. You just do what you do. The last debate I had two on one. I usually have two on one at least. And I did well in the last debate and we did well with Savannah Guthrie based on reports, but all you can do is look, you know, you go around. We do interviews with you. This is like -- I call this debate prep."

The president also told reporters he'd be fine with authorizing his doctors to release confirmation that he's been tested for COVID-19 before this week's debate in Nashville, following Biden calling it a matter of "decency" at an ABC News Town Hall last week.

"Sure, I would have no problem with it. I’m -- not only am I free -- positive or whatever, what would you call it. I'm totally free, right? Not only am I free. I'm immune, they say," he said.

Trump campaign manager Bill Stepien tweeted that he sent a letter to the bipartisan Commission on Presidential Debates demanding the third presidential debate be focused on foreign policy and also, as the campaign did on a call earlier, railed at any rule changes, including “granting an unnamed person the ability to shut off a candidates microphone.”

"The commission's pro-Biden antics have turned the entire debate season into a fiasco," Stepien said in the letter.

The letter makes no mention of the president potentially skipping the third debate if Trump's demands are not met.

The commission said last week it was looking at changes, but finals plans have yet to be announced.

Reacting to the Trump campaign’s letter to the debate commission, the Biden campaign emphasized that the decision to let moderators choose the topics was agreed upon months ago."The campaigns and the Commission agreed months ago that the debate moderator would choose the topics. The Trump campaign is lying about that now because Donald Trump is afraid to face more questions about his disastrous COVID response," Biden’s national press secretary TJ Ducklo said in a statement. "As usual, the president is more concerned with the rules of a debate than he is getting a nation in crisis the help it needs."

--ABC News' Will Steakin and Jordyn Phelps