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.”