President Donald Trump on Thursday targeted another elite law firm that has represented clients he considers his political enemies, sending a forceful message that he is willing to punish firms who work for people or groups that oppose his administration’s agenda.

In an Oval Office ceremony, the president signed an executive order hitting the large international law firm Perkins Coie with a sweeping directive that bans the federal government from hiring it, or from using contractors who work with it, except in limited circumstances. The order also bars Perkins Coie employees from entering federal buildings and suspends their security clearances.

The firm represented Hillary Clinton’s campaign and the Democratic National Committee during the 2016 presidential race, and it also contracted with the research firm that produced the now-discredited opposition dossier that alleged extensive contacts between Trump and Russia.

The move could have a chilling effect on law firms’ willingness to take on clients and cases that run counter to the Trump administration, challenging a fundamental tenet of the rule of law in the United States that everyone should have access to legal representation, experts said.

“This is an absolute honor to sign,” Trump said from his Oval Office desk.

A spokesman for Perkins Coie said in a statement that the executive order is “patently unlawful, and we intend to challenge it.”

A White House official said the move targeted Perkins Coie because of its track record on Trump.

“The president doesn’t believe they should have the privileges afforded to companies of their stature to work and operate with the federal government, since they have made it very clear they are vehemently against the president of the United States, and their work proves that,” a senior administration official said, speaking on the condition of anonymity to talk frankly about the sensitive decision-making behind the order.

“If this case was retribution or anti-Democrat, anti-liberal, then we would basically pull the security clearance clearances of every law firm on K Street, because they all hate the president,” the official said. “This law firm in particular has shown a level of bias in which their coordination with the federal government was done so in an illegal fashion that was purposely intended on undermining the president of the United States and really locking him up.”

Perkins Coie is the second law firm Trump has taken action against or decried as engaging “in the weaponization of the judicial process.”

Last week, he ordered the suspension of security clearances of some attorneys at Covington & Burling, which has represented special counsel Jack Smith pro bono. Smith led the federal investigations of Trump for allegedly mishandling classified materials and trying to block the 2020 election results.

Trump’s orders targeting the law firms called for limiting their access to federal resources and could have the effect of dissuading other private law firms from battling the Trump administration in court.

The American legal system relies on attorneys representing all sorts of clients, including violent criminals, and the president’s appearing to punish firms for their choice of clients could do grave damage to that system, said Mark Zaid, a national security lawyer who often represents whistleblowers.

“By taking these actions against these major law firms, it is effectively sending a message to all the lawyers that you best not challenge this administration, or you, your lawyers and your clients will suffer,” Zaid said. “It is the most un-American thing I’ve ever heard.”

Other experts said the order was part of Trump’s broader campaign against the legal system.

“He doesn’t like judges who rule against him. He doesn’t like prosecutors who prosecuted against him. He doesn’t like lawyers who’ve undertaken measures against him,” said Thomas Carothers, the director of the Democracy, Conflict, and Governance Program at the Carnegie Endowment for International Peace. “This is part of a broader questioning, and in many cases, an attack on the core institutions of the rule of law.”

But Carothers said that he did not think Trump’s order would bring the U.S. legal system to a halt.

“I don’t think a peer law firm will think, ‘Oh, we shouldn’t bring this enforcement action against this government entity, because we can no longer challenge the U.S. government.’ I think that would be overreading it.”

Perkins Coie became embroiled in what Trump and his allies have repeatedly described as the “Russia hoax” when it contracted with a firm that conducted research about Trump’s connections to Russia during the 2016 presidential campaign. That contract resulted in the now-famous Steele dossier, a document full of unverified allegations assembled by former British intelligence officer Christopher Steele.

But the two main attorneys involved in that work — Marc E. Elias and Michael Sussmann — are no longer employed by Perkins Coie. Both attorneys are named in a fact sheet explaining Trump’s executive order.

Elias retained Fusion GPS, the Washington firm that ultimately produced the Steele dossier.

Years later, special counsel John Durham — who was assigned to look into possible wrongdoing among federal agents who investigated Trump’s 2016 campaign — charged Sussmann with lying to a senior FBI official during that investigation.

Sussmann was accused of bringing allegations to the FBI of a secret computer communications channel between the Trump organization and Russia-based Alfa Bank. FBI agents investigated the data but concluded that there was nothing suspicious about it.

Perkins Coie is one of eight big law firms involved in lawsuits against Trump, according to an American Bar Association Journal article published last week that cited the law firm’s representation of transgender service members who are challenging the administration’s ban on joining the military.

Trump also signed a separate order that could make it harder for opponents to seek injunctions against his administration’s actions by holding them accountable for legal costs if they lose. The Justice Department will ask judges to require plaintiffs who are seeking injunctions and restraining orders against the Trump administration to post a bond to cover court costs should they fail, raising the bar for legal challenges.

When presenting the Perkins Coie executive order to Trump to sign, White House staff secretary Will Scharf said the firm has engaged in “unlawful DEI practices,” referring to diversity, equity and inclusion programs, and accused it of using racial quotas for hiring and promotions.

Scharf said the executive order calls for a “holistic review of unlawful DEI in some of the nation’s largest law firms.”

Although the Supreme Court has banned affirmative action in college admissions, and federal law prohibits workplace discrimination on the basis of race, sex and certain other criteria, no law bans diversity and equity efforts within private companies.

Scharf did not explain what the government’s review would entail, which law firms would be examined and what criteria the federal government would use to determine whether a law firm engaged in wrongdoing.

The Trump administration’s move to penalize law firms for their DEI practices when no court has said such programs are illegal “is extraordinary,” said Walter Olson, a senior fellow at the Cato Institute, a libertarian think tank.

“There is no legal definition of excessive DEI. The law itself is so unsettled,” Olson said. “The administration can take positions, but it’s not as if the courts have taken a position and given them a green light and said, ‘Yes, you are right.’”

An earlier version of this article misspelled the last name of a former Perkins Coie attorney. He is Michael Sussmann. This article has been corrected.