GEOFF BENNETT: In a matter of days, one of the most popular social media apps in the country could be forced to pull the plug.

That's if the Supreme Court doesn't grant it a legal reprieve.

John Yang is here to explore TikTok's legal battle and its looming deadline to cut its ties with China.

JOHN YANG: Geoff, while the justices heard arguments over the looming U.S. TikTok ban inside the court, outside, some influencers gathered on the steps to livestream their support for the platform.

There are more than 170 million TikTok users in the United States.

That's about half the population.

In last year's presidential campaign, both candidates used it to court young voters.

But concerns about its Chinese ownership led Congress to pass a law that would ban it in the United States on January 19 unless its parent company sells it.

Bobby Allyn covers technology for NPR.

And Marcia Coyle is PBS News Supreme Court analyst.

Bobby, let me start with you.

What does this law actually do on the 19th?

The TikTok lawyer today kept saying that TikTok would go dark.

Is that an overstatement?

BOBBY ALLYN, Business and Technology Reporter, NPR: Yes, on January 19, Apple and Google will be legally forced to remove TikTok from app stores.

And that means new users can't download it and ByteDance in China will not be able to send the app software updates.

Also, Web hosting services that provide back-end support for TikTok will have to sever ties.

That means it's not going to have any cloud support.

That's going to be a huge problem for all of the people taking videos, commenting, trying to use TikTok, because basically it will cripple TikTok's infrastructure.

JOHN YANG: And, Marcia, in the arguments today, what were the justices focusing on?

MARCIA COYLE: John, they were really focused on the two reasons that Congress and the administration have given to justify the law, as Bobby just stated it.

That had to do with the fact that the Chinese government, through its very close alignment with ByteDance, the China-based owner of TikTok, may engage in covert content manipulation, as well as its collection of the data of private citizens, Americans, about 177 million Americans in their contacts, and use both of those to undermine national security.

JOHN YANG: There was also a First Amendment argument, wasn't there?

MARCIA COYLE: Well, actually, the guts of this case is whether the law itself violates the First Amendment speech rights of TikTok USA and the users of TikTok.

They're called creators, those who put content up on TikTok.

And, today, during the arguments, it was very interesting, because there was a lot of skepticism about the first justification, the Chinese government engaging in manipulation, covert manipulation, of content, and whether there actually was a speech interest here to look at.

As some of the justices said, and I will point in particular to Chief John Roberts, he said Congress didn't care about the expression on TikTok, meaning the speech or the ideas on TikTok.

Congress didn't want to stop TikTok, he said.

What Congress wanted to do was to stop China's control of TikTok.

So he was very skeptical that there was a speech right here.

Also, there were justices who did recognize there were speech rights belonging to TikTok USA and the users of TikTok, but they questioned - - for example, Justice Elena Kagan questioned whether that -- those speech impediments or restrictions were really substantial, because the law itself, she said, really was targeted at ByteDance and its divestiture.

JOHN YANG: Were the justices more skeptical of one side or the other?

Did they tip their hand at all about what they might do?

MARCIA COYLE: Well, I think they were very tough on both sides, but I think what resounded most with the justices was that second interest that the government offered for the law, the collection of the private information of American citizens.

Justice Brett Kavanaugh, for example, he said, that's a huge concern now and in the future.

As the government pointed out, that data, private data, could be used by China in the future to blackmail future CIA officers.

It could be used to try to turn certain Americans into spies.

And so I think, when you looked overall at the arguments, that was something that may persuade a majority to very much uphold this law.

JOHN YANG: And, Bobby, if that happens, if this law is upheld, is there any indication what TikTok and its owners would do as the 19th approaches?

BOBBY ALLYN: That's really the million-dollar question that we don't know.

TikTok and its parent company ByteDance are in this really precarious amount of uncertainty right now.

If the law is upheld, president-elect Donald Trump will not yet be in office.

And between January 19, the start date, and when Trump is sworn in, the 20th, there's going to be 24 hours of limbo.

And I have sources inside of Google and inside of Apple who say, look, we have heard from our general counsels that we're not about to be out of compliance with a federal law on the books.

So they are planning, the two big tech companies, to remove TikTok on the 19th.

And I think a big question is, even if the law is upheld, will Trump then -- can he, rather, extend the deadline?

How do you -- this came up in the court today, right?

How do you extend the deadline on a ban that has already started?

So lots of unknowns, but it could get really serious really fast for TikTok.

JOHN YANG: And, actually, at the end of last year, the president -- the president-elect filed a brief in the court asking them to delay this so that he could have time to try to negotiate a deal, as he put it.

And is there a chance that he just might -- if the law is upheld, he just might not enforce it?

BOBBY ALLYN: That's a possibility.

Trump, remember, was the person who started the TikTok ban movement during his first term, but now coming into office, he wants to be TikTok's savior.

And it is within the realm of possibility that he instructs his Justice Department to not enforce this law.

If you look at the language of the law Congress passed, it puts a lot of leeway in the hands of the president.

The president will be the one who will be interpreting and instructing his administration to enforce the law.

So a lot will come down to what is Trump going to do once he's in office, regardless of what the High Court decides.

JOHN YANG: Bobby Allen of NPR, Marcia Coyle, our Supreme Court analyst for PBS News, thank you both very much.

BOBBY ALLYN: Thanks for having me.

MARCIA COYLE: Thanks, John.