TikTok and its Communist Chinese parent company ByteDance are suing America over a law that would ban the popular video-sharing app unless it’s sold to another company, arguing that it relies on vaguely painting it as a threat to national security to get around the First Amendment.

The widely expected lawsuit filed on Tuesday may be setting up what will likely be a protracted legal fight over TikTok’s future in the United States — and could end up before the Supreme Court. If TikTok loses, it says it will be forced to shut down next year.

The popular social video company says that the law, which President Biden signed as part of a larger $95 billion foreign aid package, is so “obviously unconstitutional” that the sponsors of the measure are trying to portray it not as a ban, but as a regulation of TikTok’s ownership. It’s the first time the U.S. government has singled out a social media company with a potential ban, which free speech advocates note is more common in repressive regimes such as Iran or China.

“Congress has taken the unprecedented step of expressly singling out and banning TikTok: a vibrant online forum for protected speech and expression used by 170 million Americans to create, share, and view videos over the Internet,” ByteDance said in its suit, filed in a federal appeals court at Washington D.C. “For the first time in history, Congress has enacted a law that subjects a single, named speech platform to a permanent, nationwide ban, and bars every American from participating in a unique online community with more than 1 billion people worldwide.”

The law requires TikTok’s parent, ByteDance, to sell the platform within nine months. If a sale is already in progress, the company will get another three months to complete the deal. ByteDance has said it “doesn’t have any plan to sell TikTok.” Only even if it wanted to divest, the company would have to get a blessing from Beijing. According to the lawsuit, the Chinese government has “made clear” that it would not permit ByteDance to divest the recommendation engine that is “key to the success of TikTok in the United States.”

TikTok and ByteDance argued in the lawsuit that it isn’t being given a choice.

“The ‘qualified divestiture’ demanded by the Act to allow TikTok to continue operating in the United States is simply not possible: not commercially, not technologically, not legally,” they said.

“There is no question: the Act will force a shutdown of TikTok by January 19, 2025,” the lawsuit says.

The lawsuit argues that it would be impossible for ByteDance to divest its American TikTok platform as a separate entity from the rest of TikTok, which has 1 billion users worldwide — most of them outside of America. A U.S.-only TikTok, the suit says, would operate as an island that’s detached from the rest of the world.

TikTok also paints divestment as a technological impossibility, since the law requires all of TikTok’s millions of lines of software code to be wrested from ByteDance so that there is no “operational relationship” between the Chinese company and the new American app.

“Specifically, to comply with the law’s divestiture requirement, that code base would have to be moved to a large, alternative team of engineers — a team that does not exist and would have no understanding of the complex code necessary to run the platform,” the lawsuit says.

The parties argued that they should be protected by the First Amendment’s guarantee of freedom of expression. They are seeking a declaratory judgment that the Act violates the American Constitution; an order enjoining Attorney General Garland from enforcing the Act and any further relief that the court may deem appropriate.

The Justice Department declined to comment on the suit Tuesday.

ByteDance will first likely ask a court to temporarily block the federal law from going into effect, a senior fellow at the University of Pennsylvania’s Carey Law School, Gus Hurwitz, said. And the decision whether to grant such a preliminary injunction could decide the case, he said. In its absence, he said, “ByteDance is going to need to sell TikTok before this case is ever decided.”

Whether a court will grant such an injunction remains unclear to Mr. Hurwitz, largely because it requires balancing important freedom-of-speech issues against the Biden administration’s claims of harm to national security. “I think the courts will be very deferential to Congress on these issues,” he said.

The fight over TikTok takes place as America-China relations have shifted to that of intense strategic rivalry, especially in areas such as advanced technologies and data security, seen as essential to each country’s economic prowess and national security.

American lawmakers from both parties, as well as administration and law enforcement officials, have expressed concerns that Chinese authorities could force ByteDance to hand over American user data or sway public opinion by manipulating the algorithm that populates users’ feeds. Some have also pointed to a Rutgers University study that maintains TikTok content was being amplified or underrepresented based on how it aligns with the interests of the Chinese government, which the company disputes.

Opponents of the law argue that Chinese authorities — or any nefarious parties — could easily get information on Americans in other ways, including through commercial data brokers that rent or sell personal information. They note the American government hasn’t provided public evidence that shows TikTok sharing American user information with Chinese authorities, or tinkering with its algorithm for China’s benefit.

The executive director of the Knight First Amendment Institute at Columbia University, Jameel Jaffer, expects TikTok’s challenge to the ban to succeed.

“The First Amendment means the government can’t restrict Americans’ access to ideas, information, or media from abroad without a very good reason for it—and no such reason exists here,” Mr. Jaffer said in a printed statement.

While TikTok has prevailed in its earlier First Amendment challenges, it is not clear whether the current lawsuit will be as simple.

“The bipartisan nature of this federal law may make judges more likely to defer to a Congressional determination that the company poses a national security risk,” a law professor and associate director of the First Amendment Clinic at Cornell University, Gautam Hans, said. “Without public discussion of what exactly the risks are, however, it’s difficult to determine why the courts should validate such an unprecedented law.”