First Amendment expert Floyd Abrams said Sunday that CNN “should sue” the White House for revoking the press credentials of correspondent Jim Acosta.

It would be a “really strong lawsuit,” the attorney told Brian Stelter on CNN’s “Reliable Sources.” Abrams referred to a case in which the White House revoked credentials in the 1970s. The U.S. Court of Appeals for the District of Columbia Circuit ruled then that the White House must provide notice in such a case, as well as an opportunity for a journalist to respond and a written opinion from the White House explaining its reasoning that a court can examine.

“We’ve had none of those things here,” Abrams explained.

“I can understand CNN being reluctant to sue because the president keeps saying CNN is the enemy of me, and CNN might have some reluctance to have a lawsuit titled CNN v. Donald Trump,” he said. “That said, yes, I think they should sue.”

Abrams is one of the foremost experts on the First Amendment and has argued several cases on free speech rights before the Supreme Court.

“This is going to happen again,” Abrams warned about the White House revoking press credentials. “So whether it’s CNN suing or the next company suing, someone is going to have to bring a lawsuit. And whoever does is going to win unless there’s some sort of reason.”

Former ABC White House reporter Sam Donaldson said Sunday that he has been asked to prepare an affidavit to support a case by CNN. He said the president’s decision to yank Acosta’s credentials is “not only wrong and unfair; it’s dangerous for the press as a whole.”

A CNN spokesperson told HuffPost that “no decisions have been made” on whether to file a lawsuit. “We have reached out to the White House and have gotten no response,” the spokesperson added.

Earlier, a CNN statement said that the Trump administration’s treatment of Acosta was “retaliation” for coverage of Donald Trump that the president does not like.

Acosta’s White House credentials were revoked last week after a furious Trump refused to answer a follow-up question from Acosta at a press conference, told the journalist “that’s enough” and called him a “terrible person.”

White House press secretary Sarah Huckabee Sanders justified the decision by saying Acosta had “put his hands on” an intern as she tried to grab for the microphone the journalist was holding, which Acosta denied. Video of the event appears to show Acosta accidentally grazing the intern’s arm as he gesticulates toward Trump. Acosta immediately apologized, saying, “Pardon me, ma’am.”

A video apparently doctored by a contributor to the right-wing conspiracy website Infowars was posted by Sanders on Twitter as “evidence” of her claim. The clip speeds up Acosta’s hand gesture, making it appear forceful. Top White House adviser Kellyanne Conway confirmed Sunday that the video was “sped up,” saying that such altering is “done all the time” on sports programs.

Acosta’s “pardon me” was not included in the White House version of the video.

At the press conference, Acosta first asked the president why he thought the caravan of migrants heading north through Mexico to the U.S. border was an “invasion.” Trump responded that it’s an invasion “because I consider it an invasion.”

Check out Abrams’ comments on the situation in the video above. His remarks begin at 2:20.