WASHINGTON — A coalition of civil liberties and human rights groups urged the Biden administration on Monday to drop efforts to extradite the WikiLeaks founder Julian Assange from Britain and prosecute him, calling the Trump-era case against him “a grave threat to press freedom.”

The coalition sent a letter urging a change in course before a Friday deadline for the Justice Department to file a brief in a London court. American prosecutors are due to explain in detail their decision — formally lodged on Jan. 19, the last full day of the Trump administration — to appeal a ruling blocking their request to extradite Mr. Assange.

The litigation deadline may force the new administration to confront a decision: whether to press on with the Trump-era approach to Mr. Assange, or to instead drop the matter.

Democrats like the new Biden team are no fan of Mr. Assange, whose publication in 2016 of Democratic emails stolen by Russia aided Donald J. Trump’s narrow victory over Hillary Clinton. But the charges center instead on his 2010 publication of American military and diplomatic documents leaked by Chelsea Manning, and they raise profound First Amendment issues.

“The indictment of Mr. Assange threatens press freedom because much of the conduct described in the indictment is conduct that journalists engage in routinely — and that they must engage in in order to do the work the public needs them to do,” the letter said, adding: “News organizations frequently and necessarily publish classified information in order to inform the public of matters of profound public significance.”

The Freedom of the Press Foundation organized the letter. Other signers — about two dozen groups — included the American Civil Liberties Union, Amnesty International USA, the Center for Constitutional Rights, the Committee to Protect Journalists, Demand Progress, the Electronic Frontier Foundation, Human Rights Watch, the Knight First Amendment Institute at Columbia University, the Project on Government Oversight and Reporters Without Borders.

“Most of the charges against Assange concern activities that are no different from those used by investigative journalists around the world every day,” Kenneth Roth, the executive director of Human Rights Watch, said in a separate statement. “President Biden should avoid setting a terrible precedent by criminalizing key tools of independent journalism that are essential for a healthy democracy.”

For now, the Justice Department remains committed to appealing the denial of its request to extradite Mr. Assange, said Marc Raimondi, a spokesman for its National Security Division.

The deadline to either continue working to extradite Mr. Assange by filing the brief or drop the matter reflects a common legal policy dilemma when a new administration takes over and confronts matters inherited from its predecessor. Newly installed officials face too many issues to make careful decisions on all at once, so some get punted.

But litigation calendars can force early decisions about whether to proceed or shift direction in some cases. It is often easier to stay the course, based on an argument that the issue can be revisited later when there is more time. But once the new administration has started down that path, it owns the policy as a matter of political and bureaucratic reality and so can effectively get locked in.

Complicating matters for making any decision to keep or jettison the Trump-era policy to go after Mr. Assange with criminal charges, the Biden administration’s intended leadership team is not yet in place at the Justice Department. The Senate has yet to confirm Mr. Biden’s nominee to be attorney general, Judge Merrick B. Garland.

In the meantime, the department is being temporarily led by a caretaker career official, Monty Wilkinson, the acting attorney general to whom the letter was addressed.

After Mr. Assange published the documents provided by Ms. Manning in 2010, the Obama administration engaged in extensive deliberations under Attorney General Eric H. Holder Jr. over whether to prosecute Mr. Assange but never charged him with a crime.

By contrast, Ms. Manning, a low-level Army intelligence analyst who downloaded the archives of documents and sent them to WikiLeaks, was convicted at a court-martial trial in 2013 of leaking the documents and sentenced to 35 years in prison. President Barack Obama commuted most of the remainder of her sentence in 2017.

But law enforcement officials under Mr. Obama shied away from bringing charges against Mr. Assange. They feared that there was no legally meaningful way to distinguish his actions from those of conventional investigative national-security journalism as practiced by mainstream news organizations like The New York Times. The Obama team did not want to create a precedent that could chill or cripple traditional journalism, according to people familiar with its deliberations.

In March 2018, however, under Attorney General Jeff Sessions, the Trump Justice Department obtained a grand jury indictment against Mr. Assange. It initially sidestepped press freedom issues by narrowly accusing him of participating in a hacking-related criminal conspiracy with Ms. Manning, rather than focusing on his publication of government secrets.

That indictment was unsealed in April 2019, when Mr. Assange was dragged out of the Ecuadorean Embassy in London and arrested. (He had taken refuge there in 2012, initially to avoid extradition to Sweden to face questions about sexual assault accusations, which he has denied. Sweden had rescinded its arrest warrant for Mr. Assange in 2017.)

The Justice Department — by then under Attorney General William P. Barr — then obtained a superseding indictment expanding the charges against Mr. Assange to include allegations that his journalistic-style activities violated the Espionage Act. A second superseding indictment later added more allegations related to the notion of a hacking conspiracy.

Notably, there is some overlap in personnel from earlier internal debates about the dilemma raised by Mr. Assange. The top national security official in the Trump Justice Department, John C. Demers, remains in place atop its National Security Division for now; the Biden transition asked him to temporarily stay on for continuity purposes even as most other Trump political appointees resigned.

Mr. Demers’s predecessor from 2013 to 2016, John Carlin, has returned to the Justice Department and is currently serving as the acting deputy attorney general. Mr. Carlin’s predecessor, Lisa O. Monaco, who ran the National Security Division from 2011 to 2013, is Mr. Biden’s nominee to be deputy attorney general but has not yet been confirmed.

The letter from the rights groups portrayed the Trump-era Justice Department’s decision to proceed against Mr. Assange as jeopardizing journalism “that is crucial to democracy” more broadly, and noted that the Trump administration had “positioned itself as an antagonist to the institution of a free and unfettered press in numerous ways.”

They added: “We are deeply concerned about the way that a precedent created by prosecuting Assange could be leveraged — perhaps by a future administration — against publishers and journalists of all stripes.”

Since the original indictment was unsealed, lawyers for Mr. Assange have fought the extradition request, arguing that the United States was prosecuting him for political reasons.

A British judge in January largely rejected those arguments, holding that he had been charged “in good faith.” But she denied his extradition anyway — citing harsh conditions for security-related prisoners in American jails and the risk that Mr. Assange might be driven to commit suicide. It is that rationale that the brief due on Friday would appeal.

Elian Peltier contributed reporting from London.