Here are some key takeaways:

It appeared the court would uphold the law.

While the justices, from across the ideological spectrum, asked tough questions of both sides, the overall tone and thrust seemed to indicate greater skepticism of the arguments made by lawyers for TikTok and its users that the First Amendment barred Congress from passing the law.

The questioning began with two conservative members of the court, Justice Clarence Thomas and Chief Justice John G. Roberts Jr., pointing out that it was not TikTok, an American company, but its Chinese parent company ByteDance that was directly affected by the law.

Another conservative, Justice Brett M. Kavanaugh, highlighted the risk that the Chinese government could use the information TikTok collects on tens of millions of American teenagers and twenty-somethings to eventually “develop spies, betray people, blackmail people.” as they grow older and go to work for national security agencies or the military.

Justice Elena Kagan, a liberal, asked why TikTok couldn’t simply create or purchase a different algorithm instead of using ByteDance’s.

And another liberal, Justice Ketanji Brown Jackson, said she believes the law is less about speech and more about association. She suggested that banning TikTok from associating with a Chinese company was like banning Americans from associating with foreign terrorist groups on national security grounds. (The Supreme Court has upheld this as constitutional.)

Still, several justices expressed skepticism about much of the government’s rationale for the law: the risk that China could “secretly” direct TikTok to manipulate the content shown to Americans or collect user data to achieve its geopolitical goals.

Both Justice Kagan and Justice Neil M. Gorsuch, a conservative, emphasized that everyone now knows that China is behind TikTok. They seemed interested in whether the government’s interest in preventing “covert” exploitation of the platform by a foreign adversary could be achieved in a less harsh way, such as by placing a label warning users of this risk.

Lawyers for TikTok and its users argued that the law was unconstitutional.

Two lawyers argued that the law violates the First Amendment: Noel Francisco, who represents both TikTok and ByteDance, and Jeffrey Fisher, who represents TikTok users. Both said concerns about possible manipulation by the Chinese government of the information American users see on the platform were not enough to justify the law.

Mr. Francisco claimed that the government of a free country “has no legitimate interest in preventing foreign propaganda” and cannot constitutionally try to stop Americans from “being persuaded by Chinese misinformation.” That targets the content of speech, which the First Amendment doesn’t allow, he said.

Mr. Fisher claimed that fears that China could use its control of the platform to promote posts that sow doubts about democracy or promote pro-China and anti-American views were a weaker justification for interfering with free speech than concerns regarding foreign terrorism.

“The government just can’t get around to saying ‘national security’ and the case is closed,” Mr Fisher said, adding: “It’s not enough to say ‘national security’ – you have to say, ‘What is it?’ real damage?'” ‘”

The Biden administration defended Congress’ right to enact the law.

Attorney General Elizabeth B. Prelogar argued that Congress had the lawful authority to enact the law and that it did not violate the First Amendment. She said it was important to recognize that the law would fully allow expression on TikTok once the platform was freed from foreign control.

“The same speech that takes place on TikTok could also take place after the divestiture,” she said. “The law doesn’t regulate this at all. So that doesn’t mean that you can’t make a pro-China speech, but that you can’t make an anti-American speech. It doesn’t regulate the algorithm.”

She added: “TikTok could, if it were able, use the exact same algorithm to show the same content to the same users. “The act simply attempts to surgically remove the ability of a foreign enemy nation to access our data and exercise control over the platform.”

The court is unlikely to wait for Trump.

President-elect Donald J. Trump has asked the Supreme Court to issue a preliminary injunction delaying the law’s effective date until he takes office on January 20.

Mr. Trump once shared the view that Chinese control of TikTok posed an unacceptable risk to national security, but changed course when he met with a billionaire Republican donor who had a stake in the parent company.

If the court upholds the law, TikTok would effectively be banned in the United States on January 19, Mr. Francisco said. He reiterated calls for the court to temporarily suspend the law’s entry into force to push back that deadline, saying it would “just give everyone a little breathing room.” It could be a “different world” for TikTok after Jan. 20, he added.

However, the judges paid little attention to this idea, suggesting that they did not take it seriously. Mr. Trump’s brief asking the court to postpone the matter beyond the end of President Biden’s term so he could address it – signed by his successor as attorney general, D. John Sauer – was a long rhetorical exercise , in which he praised Mr. Trump, but lacked substance.