The group filing papers in U.S. District Court in Pennsylvania included the American Civil Liberties Union, the NAACP, the League of Women Voters and Common Cause, saying they represent some 50,000 voters who cast ballots by mail -- a frequent target of Trump's unsubstantiated attacks on electoral system.
Benjamin Geffen, a staff attorney at the Public Interest Law Center, which is working with several of the groups, called the Trump campaign lawsuit that seeks to invalidate the election an attack on voters.
"This case asks the courts to yank away their votes," Geffen said. "It would be the ultimate case of disenfranchisement. It isn't any different than being told you can't vote in the first place."
The defendant in the Trump campaign case is the Pennsylvania secretary of the commonwealth, Kathy Boockvar, who oversaw last week's elections. But Kristen Clarke, president of the Lawyers' Committee for Civil Rights Under Law, said the voters need to defend themselves as well.
"Plaintiffs are effectively challenging the votes of millions of Pennsylvania voters and we are here to protect the rights of those voters," Clarke said.
The 86-page complaint the Trump campaign filed Monday asks the court to prohibit the state from certifying the election -- or at least the over 680,000 mail-in ballots cast in the Pennsylvania election.
The Trump case lays out a laundry list of grievances about the election, but largely centers its argument on the notion that voters in Democratic-leaning counties received more lax oversight from elections officials than Republican-leaning counties.
"While the bedrock of American elections has been transparency, almost every critical aspect of Pennsylvania's November 3, 2020 General Election was effectively shrouded in secrecy," the Trump lawsuit alleges.
The lawsuit focuses on the mail-in ballots processed in Democratic strongholds of Allegheny and Philadelphia counties. And it argues that Boockvar has "created an illegal two-tiered voting system" that subjected in-person voters to "greater burdens or scrutiny" than those who voted by mail.
That assertion -- of two different tiers of voters -- undergirds the lawsuits claim that the election violated the Constitution's Equal Protection guarantees.
But Geffen, the public interest attorney, said that argument is undermined by the fact that Pennsylvania has long deferred to counties to handle the "nitty gritty" of carrying out elections.
"The counties have for many decades made lots of independent decisions about the day to day operations of the election," Geffen said. "You don't need absolute uniformity of every detail of the election for it to be a constitutional and fair election."
The support for these claims includes a grab bag of allegations -- some of which have been previously rejected by courts, and others that rely on hearsay or suspect sources -- and have faced condemnation from legal experts in both parties.
In a call with reporters Tuesday, Bob Bauer, the lead attorney for President-elect Joe Biden, derided the Trump case as an "embarrassment." Bauer called the federal case and a host of others filed in the past week "noise" and "theatrics," and added that he is unconcerned about the president's effort to bring an election challenge to the U.S. Supreme Court.
"I don't know which one of these cases they will inflict on the Supreme Court," Bauer said. "But they won't win it."
In filings Tuesday, the civil rights and public interest groups were joined by eight individual voters who joined the case on behalf of those who cast mail-in or absentee ballots in the 2020 election, or who voted by provisional ballot or in person after receiving notice of a mail-in ballot error. Several of the individuals noted that COVID-19 motivated them to vote by mail, whether because of their age or because of underlying conditions.
Kenneth Huston, president of the NAACP Pennsylvania chapter, said his organization joined the case because "most mail-in balloting was done by African Americans and people of color," arguing that Black Americans would be predominantly affected by any ruling in favor of the Trump campaign.
Suzanne Almeida, the executive director of Common Cause Pennsylvania, told ABC News that her chapter felt compelled to intervene.
"We wanted to bring the perspective of voters," she said, "because they can get lost in the shuffle."