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As the Supreme Court was poised on Friday to announce its landmark decision on whether to uphold a law that would effectively ban TikTok in the United States, the popular Chinese-owned app and its allies launched a last-minute push to fend off the statute’s enforcement.

The company’s representatives have swarmed Capitol Hill in recent days, pressing lawmakers with TikTok’s case that the app, which is owned by ByteDance, should win a reprieve from potentially going dark on Sunday, when the law is scheduled to go into effect, three people familiar with the efforts said.

Senator Chuck Schumer of New York, the Democratic leader, recently told President Biden in a phone call that the ban would damage his legacy if it occurred on his watch, according to two people familiar with the conversation. Mr. Schumer, who said he wanted TikTok to be sold to an American owner instead of being banned, publicly confirmed on Thursday that he had pushed Mr. Biden to extend the deadline.

At the same time, President-elect Donald J. Trump — who has signaled his support for the app and will take office on Monday — has weighed signing an executive order that could effectively neuter the law.

The flurry of activity came as the Supreme Court, in an extraordinary move, strongly suggested that it would issue a decision on the law on Friday. In putting the case on an exceptionally fast track last month, the justices heard arguments only a week ago and appeared inclined to uphold the law.

“This threat of going dark on Sunday got people’s attention,” said Sarah Kreps, the director of the Tech Policy Institute at Cornell University. It seems to have “crystallized what’s really at stake.”

The potential ban stems from bipartisan legislation last year that would force app stores like Apple’s and Google’s, plus cloud computing providers, to stop distributing TikTok unless ByteDance sold it to a non-Chinese owner within 270 days. Mr. Biden signed the law in April because of national security concerns that the Chinese government could use the app to spy on Americans and spread propaganda.

If the Supreme Court upholds the law, as expected, the ruling will have untold repercussions for the social media landscape, popular culture, and millions of influencers and small businesses that rely on the platform to earn a living. TikTok, which says it has 170 million U.S. users, has become a cultural hub that has hosted everything from videos on dance trends to important information for communities in crisis. Other social media companies most likely stand to profit.

A ban would also have major implications for U.S.-Chinese relations, already deeply strained over trade and tariffs. Beijing restricted the export of critical minerals to the United States last month after officials in Washington placed new curbs on China’s computer chip industry. Mr. Trump has threatened a wider trade war after taking office.

It’s unclear if an executive order by Mr. Trump could effectively halt a ban. Legal experts said he could direct the Justice Department not to enforce the law, or delay enforcement for a set period, but how such an executive order would fare if challenged in court is uncertain.

Another possible way to halt a ban would be to find a buyer, allowing the president to extend the deadline by 90 days. The law says a viable deal must be on the table, but no clear buyers have emerged. Some lawmakers urged Mr. Biden or Mr. Trump on Thursday to extend the deadline anyway.

TikTok has claimed that a sale is impossible because TikTok is a global operation, and China has already signaled it would block the export of its all-important video-recommendation technology.

Instead, TikTok has been working political connections to avoid losing its U.S. business. TikTok’s chief executive, Shou Chew, has been living in the United States instead of his usual base, Singapore, amid the political chaos surrounding the company, two people with knowledge of his plans said.

Mr. Trump has invited Mr. Chew to attend the inauguration on Monday and to sit in a position of honor on the dais, where former presidents, family and other important guests traditionally are seated.

TikTok has also leaned into its connections with conservative influencers. It spent thousands of dollars to be the headline sponsor of a party this Sunday called “The Power 30,” which will honor conservative influencers.

Inside the company, employees have weighed whether the experience of using the app should change in some way if the ban takes effect.

On Friday, the Supreme Court will issue at least one opinion starting at 10 a.m., a day on which it hadn’t previously been expected to rule. The justices will not take the bench, an unusual move for a decision in argued cases.

The last time the court did so was in March, when it issued a decision in another expedited and consequential case, rejecting a challenge to Mr. Trump’s eligibility for Colorado’s primary ballot.

While the TikTok law won bipartisan support last year, some politicians now worry that a ban could trigger widespread political backlash among Americans. Many young adults rely on TikTok for an endless stream of memes, snappy dances, political commentary, news and humor.

“We will put measures in place to keep TikTok from going dark,” former Representative Mike Waltz, who is Mr. Trump’s incoming national security adviser, said Thursday morning on “Fox and Friends.” “It’s been a great platform for him and his campaign to get his America first message out, but at the same time he wants to protect their data.”

Democrats expressed worry in recent days that the app would go offline on Mr. Biden’s watch, even announcing legislation on Tuesday to extend the deadline for a sale. Mr. Schumer accused Republicans of blocking the proposal.

“It’s clear that more time is needed to find an American buyer and not disrupt the lives and livelihoods of millions of Americans, of so many influencers who have built up a good network of followers,” Mr. Schumer said during his speech on the Senate floor on Thursday.

Two of the bill’s sponsors, Senators Edward J. Markey of Massachusetts and Cory Booker of New Jersey, both Democrats, called on Mr. Biden to extend the deadline by 90 days at a news conference on Thursday — even with no deal to sell the company on the table.

“He’s the incumbent president, and he can exercise that authority right now,” Mr. Markey said.

“We hope that if President Biden doesn’t exercise that authority, that President Trump, after he is sworn in on Monday, that he would exercise that authority to ensure that TikTok stays alive,” Mr. Markey added.

Catie Edmondson contributed reporting.