Washington — The 170 million TikTok users in the U.S. could be in for a rude awakening come Sunday if they suddenly find the enormously popular video-sharing app is inaccessible because of a law passed by a bipartisan majority in Congress last year.

Lawmakers and U.S. officials have sounded the alarm for years about the supposed risks that TikTok's ties to China pose to national security, and Congress moved last year to force TikTok's Chinese parent company, ByteDance, to sell its stake in the app or be cut off from the U.S. market. The law gave the company a deadline of Jan. 19 — one day before a new president would take office.

That deadline is now here, with no sign of a sale in sight. TikTok's last-ditch legal challenge failed on Friday, when the Supreme Court said the law does not violate the First Amendment.

The Biden White House said it will leave enforcement of the law to the incoming Trump administration, and President-elect Donald Trump has vowed to "save" the app. But TikTok has hinted that it could still take itself offline once the law is in effect, a move that would leave content creators and users in the lurch as the company seeks a way to get back on firm legal footing.

In a statement provided to CBS News Friday evening, TikTok said that "the statements issued today by both the Biden White House and the Department of Justice have failed to provide the necessary clarity and assurance to the service providers that are integral to maintaining TikTok's availability to over 170 million Americans. Unless the Biden Administration immediately provides a definitive statement to satisfy the most critical service providers assuring non-enforcement, unfortunately TikTok will be forced to go dark on January 19."

Here's what to know about the TikTok ban and how we got here:

Why did Congress want to ban TikTok?

U.S. officials have repeatedly warned that TikTok threatens national security because the Chinese government could use it as a vehicle to spy on Americans or covertly influence the U.S. public by amplifying or suppressing certain content.

The concern is warranted, they said, because Chinese national security laws require organizations to cooperate with intelligence gathering. FBI Director Christopher Wray told House Intelligence Committee members last year that the Chinese government could compromise Americans' devices through the software.

As the House took up the divest-or-ban law in April 2024, Rep. Michael McCaul, a Texas Republican, compared it to a "spy balloon in Americans' phones." Sen. Chris Coons, a Democrat from Delaware, said that lawmakers learned in classified briefings "how rivers of data are being collected and shared in ways that are not well-aligned with American security interests."

"Why is it a security threat?" Republican Sen. Josh Hawley of Missouri said Friday. "If you have TikTok on your phone currently, it can track your whereabouts, it can read your text messages, it can track your keystrokes. It has access to your phone records."

If the Chinese government gets its hands on that information, "it's not just a national security threat, it's a personal security threat," Hawley said.

In 2022, TikTok began an initiative known as "Project Texas" to safeguard American users' data on servers in the U.S. and ease lawmakers' fears. The Justice Department said the plan was insufficient because it still allowed some U.S. data to flow to China.

Though the divest-or-ban law passed with bipartisan support, some lawmakers have been critical of the measure, agreeing with TikTok that it infringes on Americans' free speech rights.

"Most of the reasons the government banned it were based on accusations, not proof," Republican Sen. Rand Paul of Kentucky said Friday. "[TikTok has] never been tried and found guilty of sharing information with the communist government."

Others have changed their tune as the deadline for a ban neared, including Trump, who tried to ban the app with an executive order during his first term that was struck down in the courts.

"The irony in all of this is that Donald Trump was the first one to point out there's a problem," Sen. Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee, said Thursday. Warner said the Trump administration "did a great job of convincing me and overwhelming members of Congress" about the risks.

TikTok has its day at the Supreme Court

During arguments before the Supreme Court on Jan. 10, TikTok's lawyer did not deny the potential national security risks as the justices appeared critical of the company's legal challenge.

"I think Congress and the president were concerned that China was accessing information about millions of Americans, tens of millions of Americans, including teenagers, people in their 20s, that they would use that information over time to develop spies, to turn people, to blackmail people, people who a generation from now will be working in the FBI or the CIA or in the State Department," Justice Brett Kavanaugh said. "Is that not a realistic assessment by Congress and the president of the risks here?"

Noel Francisco, who represented TikTok, responded, "I'm not disputing the risks. I'm disputing the means that they have chosen."

Solicitor General Elizabeth Prelogar asserted that TikTok collects "unprecedented amounts" of personal data that would be "incredibly valuable" to the Chinese government by giving it "a powerful tool for harassment, recruitment and espionage."

"For years, the Chinese government has sought to build detailed profiles about Americans, where we live and work, who our friends and coworkers are what our interests are and what our vices are," she said, citing major data breaches that the U.S. has attributed to China over the last decade, including the hack of the Office of Personnel Management that compromised the personal information of millions of federal employees.

The Supreme Court's TikTok decision

In defending the law before the Supreme Court, the Justice Department pointed to two main national security justifications: countering China's collection of data from TikTok's 170 million U.S. users and its purported ability to manipulate content on the app to further its geopolitical interests.

The Supreme Court's unanimous ruling hinged on the first justification: that China, through the app and its parent company, Beijing-based ByteDance, can amass vast amounts of information from American users. The justices found that Congress did not violate the First Amendment by taking action to address that threat. Congress, it said, "had good reason to single out TikTok for special treatment."

The court refrained from backing the government's interest in stopping China's purported covert manipulation of content, which the Biden administration had cited as a national security justification for the law.

"One man's 'covert content manipulation' is another's 'editorial discretion,'" Gorsuch wrote in an opinion concurring in judgment. "Journalists, publishers, and speakers of all kinds routinely make less-than-transparent judgments about what stories to tell and how to tell them. Without question, the First Amendment has much to say about the right to make those choices."

Grace Kazarian, Alan He and Melissa Quinn contributed to this report.