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The Supreme Court will hear arguments on Friday on the fate of TikTok, the enormously popular video app that Congress says poses a looming threat to the nation’s security.

Unless the justices intervene before a Jan. 19 deadline set by a federal law, the app must be sold or shut down.

The law, enacted in April with broad bipartisan support, said urgent measures were needed because TikTok’s corporate parent, ByteDance, was effectively controlled by the Chinese government, which could use the app to harvest sensitive information about Americans and to spread covert disinformation.

Saying that the law violates both its First Amendment rights and those of its 170 million American users, TikTok has urged the court to strike down the law.

The court has put the case on an exceptionally fast track, and it is likely to rule by the end of next week. Its decision will be among the most consequential of the digital age, as TikTok has become a cultural phenomenon powered by a sophisticated algorithm that provides entertainment and information touching on nearly every facet of American life.

“Americans use TikTok to communicate about all manner of topics — from culture and sports, to politics and law, to commerce and humor,” lawyers for the app told the justices. “For instance, people of diverse faiths use TikTok to discuss their beliefs with others. Recovering alcoholics and individuals with rare diseases form support groups. Many also use the platform to share videos about products, businesses and travel.”

The Supreme Court has repeatedly taken up cases on the application of free speech principles to giant technology platforms, though it has stopped short of issuing definitive rulings. It has also wrestled with the application of the First Amendment to foreign speakers, ruling that they are generally without constitutional protection, at least for speech delivered abroad.

A three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit in early December rejected a challenge to the law, ruling that it was justified by national security concerns.

“The First Amendment exists to protect free speech in the United States,” Judge Douglas H. Ginsburg wrote for the majority, joined by Judge Neomi Rao. “Here the government acted solely to protect that freedom from a foreign adversary nation and to limit that adversary’s ability to gather data on people in the United States.”

In a concurring opinion, Chief Judge Sri Srinivasan acknowledged that under the law’s ban, “many Americans may lose access to an outlet for expression, a source of community and even a means of income.”

“Congress judged it necessary to assume that risk,” he wrote, “given the grave national security threats it perceived. And because the record reflects that Congress’s decision was considered, consistent with longstanding regulatory practice, and devoid of an institutional aim to suppress particular messages or ideas, we are not in a position to set it aside.”

ByteDance has said that more than half of the company is owned by global institutional investors and that the Chinese government does not have a direct or indirect ownership stake in TikTok or ByteDance.

The government’s brief acknowledged that ByteDance is incorporated in the Cayman Islands but said that its headquarters are in Beijing and that it is primarily operated from offices in China.

The deadline set by the law falls one day before the inauguration of President-elect Donald J. Trump. In an unusual brief last month, nominally in support of neither party, he asked the justices to temporarily block the law so that he could address the matter once in office.

“President Trump opposes banning TikTok in the United States at this juncture,” the brief said, “and seeks the ability to resolve the issues at hand through political means once he takes office.”