A Washington-based tech group supported by Facebook, Google and Twitter filed a lawsuit against President Trump on Tuesday, alleging that his executive order targeting social media giants threatens to “curtail and chill constitutionally protected speech” during the presidential election.

The challenge brought by the Center for Democracy and Technology (CDT) marks the first major legal test of Trump’s directive, which paves the way for federal agencies to investigate and penalize some of Silicon Valley’s most popular platforms over the way they police politically oriented posts, photos and videos across the Web.

Trump issued the controversial directive last week, just days after Twitter took the rare step of fact-checking one of his tweets. The president blasted the move as political censorship, accusing the social media company of something it and other major technology companies long have denied.

In its lawsuit, the CDT said the White House had run afoul of the First Amendment, which “prohibits government officials from using government power to retaliate against an individual or entity for engaging in protected speech.” Even though Trump’s order has not taken full effect, the CDT said the mere existence of the policy could “chill” speech, undermining efforts by Facebook, Google and Twitter to ensure that their platforms are used responsibly during the presidential race.

“We see the executive order as very clear retaliation that’s designed to deter social media companies from fighting misinformation and voter suppression,” said Alexandra Givens, the leader of the CDT. The group filed its lawsuit in the U.S. District Court for the District of Columbia, asking it to invalidate the whole of the order.

Facebook and Google declined to comment. Twitter praised the lawsuit in an unsigned tweet, while blasting the president’s order as “reactionary and politicized.” All three companies have given money to the CDT in the past, the group’s public statements indicate.

The White House referred requests to the Justice Department, which did not immediately respond.

The lawsuit reflects long-simmering acrimony between the Trump administration and the Silicon Valley social media sites that have become critical tools in his own political arsenal. Trump is one of the most popular, influential users on services including Facebook and Twitter, but he is also one of their most controversial, attacking critics and spreading falsehoods that might have run afoul of those companies’ rules if he did not serve as the commander in chief.

Twitter long resisted calls to discipline Trump, saying that even his most incendiary comments should be available for users to view and share without restriction. But the company took a more aggressive approach beginning last week, after Trump falsely linked mail-in ballots with election fraud. Twitter opted to append a label to his tweets, directing users to news stories that fact-checked Trump’s claims.

The president responded throughout the week by repeatedly accusing Twitter of censorship, a battery of attacks that culminated in the executive order he signed Thursday. The directive chiefly takes aim at a provision of law known as Section 230, which for decades has spared tech giants from being held liable for the content posted by their users — and the decisions those companies make about the posts, photos and videos to leave them intact or take them down.

“We’re here today to defend free speech from one of the greatest dangers,” Trump said before signing the document.

Ultimately, it is up to two independent agencies to determine how, exactly, Trump’s executive order will be implemented. A wide array of Democratic lawmakers, free-speech activists and conservative advocacy groups, however, all have condemned Trump for an order they see as unconstitutional and dangerous to the future of free expression on the Web. The CDT, for its part, called the executive order “retaliatory” in the lawsuit it filed Tuesday.

In the meantime, Trump and Twitter have continued to clash. On Friday, the president drew the company’s latest rebuke after he condemned demonstrators in Minneapolis as “THUGS,” threatened military intervention there and predicted that local looting could lead to “shooting.” In response, Twitter took the unprecedented step of limiting the public’s ability to view and share Trump’s tweet, which the company said had glorified violence.