President-elect Donald J. Trump filed an unusual brief on Friday asking the Supreme Court to block a law that requires TikTok to be sold or shut down by Jan. 19.

The deadline falls a day before Mr. Trump is to be inaugurated, and the brief asks the justices for the delay so that he may address the matter.

“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.”

The brief took no position on the legal question that the justices are set to consider when they hear arguments in the case next month: whether Congress violated the First Amendment by effectively banning TikTok. Adopting a distinctive tone at odds with the sober and measured arguments more typical in Supreme Court advocacy, the brief instead touted Mr. Trump’s expertise.

“President Trump alone possesses the consummate deal-making expertise, the electoral mandate and the political will to negotiate a resolution to save the platform while addressing the national security concerns expressed by the government — concerns which President Trump himself has acknowledged,” the brief said.

Mr. Trump, the brief added, is particularly knowledgeable about social media in general and TikTok in particular.
“President Trump is one of the most powerful, prolific and influential users of social media in history,” the brief said. “Consistent with his commanding presence in this area, President Trump currently has 14.7 million followers on TikTok with whom he actively communicates, allowing him to evaluate TikTok’s importance as a unique medium for freedom of expression, including core political speech.”

In its own brief filed Friday, TikTok told the Supreme Court that the application of the law at issue in the case violated the First Amendment by stifling the speech of 170 million American users based on mere speculation about potential Chinese national security threats.

“The government has banned an extraordinary amount of speech; demands deference to unsubstantiated predictions a future risk will materialize; and gets facts wrong when it bothers to provide them,” the brief said.

“Congress’s unprecedented attempt to single out petitioners and bar them from operating one of the nation’s most significant speech venues is profoundly unconstitutional,” the brief added.

The Biden administration responded that the law, passed with bipartisan congressional majorities, was grounded in the reality of a foreign adversary poised to gather users’ personal data and spread disinformation.

The law, the brief said, “addresses the serious threats to national security posed by the Chinese government’s control of TikTok, a platform that harvests sensitive data about tens of millions of Americans and would be a potent tool for covert influence operations by a foreign adversary.”

A brief filed on behalf of users of the service acknowledged that the United States was in the midst of an “intense geopolitical competition” with China. But the brief said that “stripping millions of Americans of their First Amendment rights” was not the right response.

“Nothing like the act here has ever been countenanced,” the brief said, “and its suppression of Americans’ speech flies in the face of our history, tradition and precedent.”

The briefs, filed on an exceptionally abbreviated schedule set last week by the justices, were the opening statements of the parties in a monumental showdown over the government’s insistence that ByteDance, TikTok’s parent company, sell the app’s operations in the United States or shut it down.

The Supreme Court, in an effort to resolve the case before the law’s Jan. 19 deadline, will hear arguments at a special session on Jan. 10.

Its ruling will decide the fate of a powerful and pervasive cultural phenomenon that uses a sophisticated algorithm to feed a personalized array of short videos to users. TikTok has become, particularly for younger generations, a compelling source of information and entertainment.

The company’s brief emphasized the outsize role the application plays.

“Americans use TikTok to communicate about all manner of topics — from culture and sports, to politics and law, to commerce and humor,” the brief said. “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 law called for expedited judicial review, giving the U.S. Court of Appeals for the District of Columbia Circuit, rather than a federal trial court, jurisdiction to hear challenges. An ideologically varied three-judge panel of that court unanimously rejected TikTok’s challenge.

Its members differed slightly in their reasoning but were united in determining that the national security justifications offered by the government were sufficient to overcome the demanding judicial scrutiny required by the First Amendment.

“The First Amendment exists to protect free speech in the United States,” Judge Douglas H. Ginsburg wrote for the majority. “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.”

Mr. Trump’s brief took aim at the deadline for the app to be sold or shut down. He is to be inaugurated on Jan. 20.

“This unfortunate timing,” his brief said, “interferes with President Trump’s ability to manage the United States’ foreign policy and to pursue a resolution to both protect national security and save a social-media platform that provides a popular vehicle for 170 million Americans to exercise their core First Amendment rights.”

The court agreed last week to hear both TikTok’s challenge, TikTok v. Garland, No. 24-656, and a separate one from creators of content on and users of the site, Firebaugh v. Garland, No. 24-657.

TikTok says it is an American company and that its corporate parent is majority-owned by global investors. That means, its brief said, that it “cannot be stripped of its First Amendment rights even (wrongly) assuming that China might be able to pressure ByteDance’s Chinese affiliates to manipulate the algorithm applied in this country.”

The government’s brief gave a different account of TikTok’s ownership and its implications. ByteDance is incorporated in the Cayman Islands, the brief said, but its headquarters are in Beijing and it is primarily operated from offices in China.

The challenged law, the government’s brief said, was a sensible response to a grave threat.

The act, the brief said, “is narrowly tailored to further the compelling interests in preventing the threats to national security posed by foreign-adversary control of TikTok: namely, the collection of sensitive data of U.S. persons and malign foreign influence of the platform targeting U.S. persons.”