WASHINGTON, Jan 17 (Reuters) - When Donald Trump ordered

the U.S. government to ban popular Chinese social media app

TikTok in 2020, he said the "aggressive action" was necessary

"to protect our national security."

Now the Republican president-elect, who will assume his

second term in the White House on Monday, is seeking to protect

TikTok from a new law that gives TikTok parent ByteDance until

Sunday to sell the app to an American buyer or be banned in the

U.S. President Joe Biden, with just three days left in office,

is being urged to give ByteDance more time to sell the app.

"We will put measures in place to keep TikTok from going

dark," Trump's incoming national security adviser, U.S.

Representative Mike Waltz, told Fox News' "Fox & Friends" on

Thursday.

So how could Trump or Biden prevent TikTok from going dark?

BIDEN TO THE RESCUE?

The TikTok law gives the president the authority to grant a

one-time extension of up to 90 days on the deadline for a sale,

if he certifies that there is a path for - and evidence of

progress toward - a divestment, including "binding legal

agreements," which the law does not define.

Meeting those requirements would make it feasible for Biden

to give ByteDance a nearly three-month reprieve, according to

Colin Costello, an attorney with Freshfields and a former

official in the Office of Director of National Intelligence. He

said the binding legal agreement criteria could potentially be

met by the signing of a simple "term sheet" between ByteDance

and a prospective buyer, although no such deal appears on the

horizon.

But to halt the ban on a longer term basis, Costello said,

could require incoming President Trump to direct his Justice

Department to "deprioritize" or not enforce the law, probably

for a specified period of time. That would take a page from

former President Barack Obama, whose administration in 2012

decided to use "prosecutorial discretion" to grant deportation

relief to immigrants who came to the U.S. illegally as children.

In TikTok's case it could give Congress time to consider a

new bill that would give ByteDance another 270 days to find an

American buyer before being shut down.

Still, the new law threatens to penalize the tech companies,

that make TikTok available via their app stores -- Apple Inc ( AAPL )

and Alphabet's Google -- and it is unclear

whether their lawyers would risk continuing to offer the app.

"That would put Apple ( AAPL ) and Google ... in a real spot," said

Costello. "Here they would have the president saying he was not

going to enforce the law although they would still be in clear

violation of the law."

TRUMP COULD GET 'CREATIVE'

Trump could issue an executive order, invoking the sweeping

International Emergency Economic Powers Act, and claim that

keeping TikTok is beneficial for national security, said Anupam

Chander, a professor at Georgetown University's Georgetown Law

School who has followed the issue.

Trump could argue, for example, that such a move would keep

users from fleeing to Chinese app RedNote, which is run directly

from China and subject to Communist Party censorship. The order

could tell tech companies like Apple ( AAPL ) or Google that they would

face no consequences for flouting the law, for example by

keeping TikTok in their app stores past the legally mandated

deadline.

Such a scenario would not be without its challenges, Chander

said.

"When the president can refuse to enforce the direct mandate

of the law and promise the subjects of the law that it will not

be enforced on them, that opens a can of worms," he said.

"I don't want to say it's illegal," he said. "It's creative

and it's an affront to Congress, but when you give the president

open-ended national security authority, he might exercise it in

unexpected ways."

SUPREME COURT TO RESCUE

The smoothest path for a TikTok rescue would be for the

Supreme Court to issue a stay in the lawsuit the app has filed

against the government, allowing the incoming Trump

administration to make a fresh argument to the court to the

effect that TikTok wasn't so bad after all.

That would still represent a dramatic break from Trump's

past attacks on TikTok, but it would avoid plunging the

government into a legal or constitutional morass.

But time is running short. The court has only one business

day between now and Sunday, when the law's divestment order

kicks in.

"The Supreme Court could still rescue President Trump's

promise to keep TikTok alive," Chander said. "We may find out

tomorrow that it's too late."