TikTok and its parent company ByteDance challenged the U.S. government in a legal filing Tuesday over a new law forcing the sale or ban of the social media giant, igniting a high-stakes court battle in Washington that could prove to be an existential fight for one of the world’s most popular apps.

President Joe Biden signed a law last month demanding that China-based ByteDance sell TikTok within a year or be banned across the United States, arguing that the Chinese government could use the app to spy on Americans or secretly shape public opinion.

But the companies in their petition for review contend that the law violates the First Amendment rights of its 170 million U.S. accounts in an “extraordinary and unconstitutional assertion of power” based on vaguely expressed national security concerns.

“Banning TikTok is so obviously unconstitutional, in fact, that even the Act’s sponsors recognized that reality, and therefore have tried mightily to depict the law not as a ban at all, but merely a regulation of TikTok’s ownership,” the challenge states.

“In reality, there is no choice,” it adds. A forced sale “is simply not possible: not commercially, not technologically, not legally.”

The case will set up a showdown for the Biden administration, which has touted the law as a way to avoid the pitfalls that doomed the Trump administration’s previous attempt to ban the app.

A loss in court for the government would set back years of backroom federal strategizing that produced one of the few national technology policies passed by Congress in two decades.

A win would force a change in control over the biggest foreign-owned tech platform to achieve mainstream prominence in the United States, disrupting what has become a potent force in the online creator economy and a popular driver of American entrepreneurship and entertainment.

ByteDance, a private company that valued itself last year at $268 billion, has vowed to fight what it calls an unconstitutional government overreach.

“We aren’t going anywhere,” TikTok chief executive Shou Zi Chew said in a TikTok video last month. “The facts and the Constitution are on our side.”

The Justice Department declined to comment.

In its 67-page petition for review of the law’s constitutionality, filed against Attorney General Merrick Garland, TikTok and ByteDance argue that the law would give the government unprecedented power to control or dismantle a speech platform it dislikes.

It also challenges the government for targeting TikTok and ByteDance by name in the law, as opposed to passing broader tech regulations for privacy or transparency that would cover the industry at large.

“If Congress can do this, it can circumvent the First Amendment by invoking national security and ordering the publisher of any individual newspaper or website to sell to avoid being shut down,” the filing says. “Congress has never before crafted a two-tiered speech regime with one set of rules for one named platform, and another set of rules for everyone else.”

The United States has yet to show evidence that China has collected TikTok’s user data for espionage or skewed its recommendation algorithm for propaganda, though the law’s sponsors have said the possibility of future influence is cause enough for aggressive action.

TikTok and ByteDance contend that the government has acted not with “any proof of a compelling interest, but on speculative and analytically flawed concerns about data security and content manipulation - concerns that, even if grounded in fact, could be addressed through far less restrictive and more narrowly tailored means.”

The companies also claim that the government did not engage seriously with TikTok’s efforts to assuage national security concerns, including its $2 billion plan, known as “Project Texas,” to safeguard Americans’ user data in a U.S. subsidiary closely overseen by federal authorities.

The companies said they made “extraordinary” commitments as part of a 90-page draft National Security Agreement that would have given the government the authority to shut TikTok down if the companies violated the agreement.

“Congress tossed this tailored agreement aside, in favor of the politically expedient and punitive approach of targeting for disfavor one publisher and speaker,” the challenge states.

Extracting TikTok from the company that has developed and managed it, the challenge says, would be infeasible given the platform’s technical underpinnings, including its globally produced library of videos, its interwoven network of data centers and its integrated systems for video editing, promotion and moderation.

China has also vowed to use its export controls to block the sale of TikTok’s backbone, its video-recommendation algorithm, which would force any new buyer to rebuild a critical component that many in Silicon Valley have spent years working to emulate.

“The platform consists of millions of lines of software code that have been painstakingly developed by thousands of engineers over multiple years,” the challenge says. “Such a fundamental rearchitecting is not remotely feasible” under a 270-day deadline.

The challenge notes a fact that it says “undermines the claim that the platform poses an actual threat to Americans”: that Biden’s campaign continues to use TikTok, even after the law was signed. The Washington Post reported on Sunday that Trump’s campaign also is discussing whether to create an account.

The challenge also seeks to use Congress members’ words against it, noting that the law’s supporters said they were also motivated by their distaste for the videos on the platform related to the Israel-Gaza war and other issues - a content-specific decision that First Amendment scholars say could weaken the government’s case.

What’s more, the challenge says, it would force the TikTok U.S. platform to “become an ‘island’ where Americans would have an experience detached from the rest of the” world.

TikTok sued the United States in 2020 and won, with a district judge in D.C. saying then-President Donald Trump’s ban order had overstepped its legal bounds. A federal judge in Pennsylvania later that year sided with TikTok creators over administration officials, and a third federal judge in Montana last year said the state’s TikTok ban “violates the Constitution in more ways than one.”

But TikTok has never fought in the court it’s stepping into: the U.S. Court of Appeals for the D.C. Circuit, the main venue for appeals of administrative and constitutional law. The law designated the high-level appeals court as the “exclusive jurisdiction” for any challenge, meaning a loss for TikTok can only be overturned by the Supreme Court.

And while Trump’s executive order was grounded in an ill-fitting 1977 law expanding the president’s power over international commerce during a national emergency, TikTok and ByteDance must grapple with a law drafted specifically with help from Justice Department officials intended to help it survive legal assault.

The companies are asking that the law’s enforcement be halted until the challenge is resolved, but such injunctions are not guaranteed. ByteDance’s current sale deadline is Jan. 19, a day before the presidential inauguration, though the government could grant a 90-day extension if it declares progress on a sale is underway.

The legal challenge is being led by Erich Andersen, a former Microsoft attorney who worked as ByteDance’s general counsel during its past legal skirmishes and represented the company in its negotiations with federal officials before the law was passed.

Underscoring the severity of the fight, ByteDance said Andersen will shift into a new role as “special counsel,” focused exclusively on driving “the company’s effort to overturn the unconstitutional ban legislation in the U.S.”

The Trump administration had appealed the district-court freeze of its TikTok ban in 2020 to the D.C. Circuit, where the new challenge will be tried. But the Biden administration, upon taking power in 2021, asked the court to dismiss the government’s appeals, saying it wanted to establish a more deliberative process than the Trump orders that one official said then hadn’t been “implemented in the soundest fashion.”

Some legal scholars said TikTok will have a solid case in arguing that the government failed to consider potential resolutions for its concerns that wouldn’t be so disruptive to Americans’ rights of free expression. But others suspect the new effort could succeed because of its firmer legal footing and the court’s traditional deference to the legislative branch on national security threats, even in cases involving restrictions of speech.

The D.C. Circuit in 2018 said Congress was within its rights to ban software by Russia-based cybersecurity firm Kaspersky Lab from federal devices, siding with the government’s claims of protecting American security and overruling the company’s complaints that the move was targeted punishment.

TikTok and ByteDance’s challenge is expected to be joined later by separate legal action from TikTok creators, who could contend, as they have in previous cases, that the government would block them from an important channel for small businesses and self-expression.

Free-speech groups are also expected to lend their support to the companies’ challenge. The Knight First Amendment Institute at Columbia University filed a brief late Monday supporting the legal challenge to Montana’s TikTok ban, writing that “the practice of restricting citizens’ access to information, ideas and media from abroad is one that has historically been associated with repressive regimes.”