The legal face-off between the Justice Department and Representative LaMonica McIver of New Jersey is set to continue in federal court this week. And the stakes are high.

Ms. McIver was charged with assaulting federal immigration agents a year ago, and on Wednesday her case will be argued before a panel of appellate judges.

To Justice Department prosecutors, the case will offer an opportunity to limit the bounds of congressional immunity.

To Ms. McIver the prosecution holds personal and political peril: up to 17 years in prison and an estimated $1 million in legal fees, just as she is running for a second term.

And to a bipartisan group of former Congress members, the charges represent an existential showdown between the executive and legislative branches.

“If the Department of Justice is allowed to proceed with this prosecution, it would create a perverse incentive for Executive Branch officials to act in a more chaotic and unsafe fashion, and create new, unprecedented tools to block legitimate legislative oversight,” wrote lawyers for the 20 former members — 17 Republicans and three Democrats — in support of Ms. McIver’s effort to have the assault charges dismissed before trial.

Ms. McIver, a Democrat and an outspoken critic of President Trump, is accused in an indictment of using her forearms to assault, resist, impede and intimidate two federal immigration agents during a May 9, 2025, clash outside Delaney Hall, a large and troubled migrant detention center in Newark. She was there with two of her Democratic House colleagues for an oversight inspection.

Her lawyers have noted that no one was injured and argued that she was being selectively prosecuted because of her politics, citing the pardons the Republican president granted to his supporters after the Jan. 6 Capitol riot, where many officers were harmed.

They have also said that she was precluded from criminal liability by a constitutional protection known as the speech-or-debate clause, which shields members of Congress from being prosecuted on charges tied to their work as legislators.

“The indictment charges her for actions she took and judgments she made while exercising her statutory and constitutional responsibilities,” her lawyers wrote.

A trial court judge, Jamel K. Semper of U.S. District Court in New Jersey, rejected both arguments.

Ms. McIver, who is 40 and pregnant with her second child, appealed, putting the case before the U.S. Court of Appeals for the Third Circuit at a time when Mr. Trump continues to show a willingness to use the Justice Department to retaliate against political adversaries.

“Members of Congress have many privileges,” wrote Mark E. Coyne, a federal prosecutor who leads New Jersey’s appeals division. “But assaulting federal agents who are trying to arrest someone isn’t one of them.”

The scope of those privileges is at the heart of the disagreement, even though there is consensus on many of the facts surrounding the brief but volatile clash, much of which was recorded.

The confrontation lasted just 68 seconds and began after agents rushed out from behind the gates of the detention center to arrest Ras J. Baraka, the city’s mayor, on a trespassing charge. Mr. Baraka, a Democrat, was running for governor at the time. And his arrest had been ordered by phone by Todd Blanche, then the deputy attorney general, whom Mr. Trump has since nominated to lead the Justice Department, according to body-worn camera footage released as part of the proceeding.

The ensuing chaos resembled a rugby scrum, both parties have noted in legal briefs.

Then, minutes after the alleged assault, Ms. McIver was invited back into the detention center to complete the oversight inspection she had arrived there to conduct.

Her lawyers have said the entire visit was core to Ms. McIver’s duties as a House member, rendering the prosecution unconstitutional.

“After re-entering the secured area,” they wrote in appeal papers, “she quickly returned to her formal tour, diligently continuing her oversight mission.”

Within two weeks, the state’s former U.S. attorney, Alina Habba, disclosed that she was dropping the trespassing charge against Mr. Baraka. In the same news release, however, she announced she was charging Ms. McIver with assault.

Prosecutors have acknowledged that others were also pushing as a crowd of roughly 40 protesters, lawmakers and reporters moved toward Delaney’s front gate alongside masked agents.

“Other protesters pushed other agents, and one agent shoved McIver; she shoved him back,” Mr. Coyne, the federal prosecutor, wrote in a legal brief.

The constitution, he stressed, does not “prohibit inquiry into illegal conduct simply because it has some nexus to legislative functions” or “immunize members of Congress who engage in violent or forcefully obstructive acts.”

“It’s the ‘speech or debate’ clause, not the ‘speech, debate or conduct’ clause or the ‘anything goes’ clause,” Mr. Coyne wrote.

The three-judge panel is expected to rule on Ms. McIver’s request to dismiss the indictment in the coming weeks.

Delaney Hall, which is the largest migrant detention center on the East Coast, has remained a flashpoint in Mr. Trump’s immigration crackdown ever since that spring day.

A month after Mr. Baraka’s arrest, four men escaped through a flimsy wall as unrest over poor conditions gripped the 1,000-bed, privately run lockup, which has a $1 billion, 15-year contract with the Immigration and Customs Enforcement agency. Detainees began a hunger strike in May, and dozens of demonstrators have been arrested outside the facility during recent protests.

“Trump and his administration want us scared and silent,” Ms. McIver said last week during a congressional hearing held in Newark’s City Hall, a few miles from the center. “They want to whitewash our streets. They want to step on the vulnerable. They want to criminalize oversight and lock me up.”

“Whatever they try, we’re not backing down,” she continued.

Regardless of what the appeals court rules, an appeal to the Supreme Court is considered possible, given the constitutional questions at the center of Ms. McIver’s argument.

Josh Chafetz, a professor at the Georgetown University Law Center who has written extensively about the separation of powers within the federal government, said the prosecution itself could have a chilling effect on lawmakers’ ability to “push back and serve as opposition” to the executive branch.

“If this winds up going to trial,” he said, “it will be a green light to presidents that they can prosecute their enemies in Congress and, indeed, that they can provoke confrontations with their enemies in Congress — and then prosecute them if they respond.”