"The Trump Campaign's present demand to set aside millions (or "potentially tens of thousands") of lawfully cast ballots -- without a single plausible factual allegation to back up this extraordinary request -- should be swiftly rejected," says the new filing on behalf of Allegheny, Philadelphia, Chester and Montgomery counties.
The case before the Third Circuit Court of Appeals appears to be on a fast track, with parties given just 24 hours to submit their arguments.
And if the ruling goes against the president -- as more than two dozen rulings have to date -- Democrats believe that one of the few remaining doors for Trump to contest the 2020 presidential contest will be shut.
A series of rulings over the past week offers signs that the Trump legal effort could soon be out of options. On Monday, the Trump campaign and its allies lost cases before both the Michigan and Pennsylvania state supreme courts. Wendy Weiser, director of the Democracy Program at the Brennan Center for Justice, said she could only draw one conclusion from the decisive Michigan decision, which rejected an effort by Trump poll observers to halt the election's certification in the state.
"The main takeaway is that this state court effort to prevent certification has reached its final unsuccessful stop," Weiser said. "It's over."
Over the weekend, a U.S. District Court judge in Pennsylvania dismissed the Trump campaign's case in federal court, which is being handled by longtime Trump ally Rudy Giuliani. The decision, which rejected outright the attempt to persuade the court to cancel millions of mail-in votes, was emphatic.
"This Court has been presented with strained legal arguments without merit and speculative accusations, unpled in the operative complaint and unsupported by evidence," Judge Matthew W. Brann wrote. "In the United States of America, this cannot justify the disenfranchisement of a single voter, let alone all the voters of its sixth most populated state. Our people, laws, and institutions demand more."
Mark Aronchick, a Philadelphia lawyer who argued the case against Giuliani, said he will wait to hear what the appeals court says before he can declare any sort of victory in the legal battle.
"I will say from the rooftops that the American public should be immensely proud that we have an independent judiciary," Aronchick said. "We operate under the rule of law, and not the rule of the soapbox from the driveway of Four Seasons Landscaping."
One person who appears not to believe the legal battle is coming to an end is President Trump. In a series of tweets in recent days, the president and his allies have pushed for ongoing efforts to challenge the results of the election in court. In one late-night tweet Monday, the president said his team is moving "full speed ahead." On Tuesday morning he promised a "big lawsuit" will be filed "soon."
At a bizarre 90-minute press conference last week, Giuliani also teased a new filing, saying "we're about to file a major lawsuit in Georgia. That will be filed probably tomorrow."
That lawsuit has yet to come, and the campaign has been absent from courtrooms in Georgia for weeks. The campaign's only case in the state came the day after the election, and it was swiftly dismissed.
Despite Trump's tweets, his team's effort in the courts has been relatively quiet. The campaign has not filed a new lawsuit since Nov. 18, and the flurry of suits filed in the days after the election have almost all concluded. Of the 19 election cases filed by the campaign, 15 have already been denied and dismissed by judges or withdrawn by the campaign as its fails to present any substantial evidence of voter fraud to back up its public claims.
Just three cases remain somewhat active. In addition to the pending appeal before the Third Circuit, the campaign has a Michigan case sitting untouched on appeal. The case was rejected as "defective" by the court for improper filing, and the issue was never corrected. Another, in Nevada, has not yet been decided, but legal experts told ABC News the case would be unlikely to impact the outcome of that state's vote count.
In the interim, several states, including Pennsylvania and Michigan, have gone forward and certified their election results. That has not changed the messaging from the Trump campaign.
"Certification by state officials is simply a procedural step," Ellis, a Trump campaign senior legal adviser, said in a statement on Monday. "We are going to continue combatting election fraud around the country as we fight to count all the legal votes."
Aronchick told ABC News that Democrats will respond if the Trump team continues to look to the courts to alter the outcome of the election.
"I would put nothing past Mr. Giuliani to figure out some other way to file somewhere else, with some other type of frivolous claim," he said. "And if they do, we'll be ready for them."