WASHINGTON (Reuters) - While President-elect Donald Trump has asked the Supreme Court to block a looming U.S. ban on TikTok in a major case being argued on Friday that pits free speech rights against national security concerns over the Chinese-owned short-video app, many of his Republican allies have urged the opposite. 

These diverging views raise the stakes for the court, which has a 6-3 conservative majority, as it prepares to decide the fate of a popular social media platform used by about half of Americans in a case testing the U.S. Constitution's First Amendment protections against government abridgment of speech.

"This is the most significant free speech case in at least a generation," said Timothy Edgar, a former U.S. national security and intelligence official who has worked in both Republican and Democratic presidential administrations. 

"If we consider that there are 170 million active monthly users of TikTok in the United States, the volume of free speech at risk is the largest of any Supreme Court case in American history," added Edgar, who now teaches cybersecurity at Brown University and joined a brief backing TikTok in the case. 

Driven by concerns that China could access data or spy on Americans with the app, Congress overwhelmingly passed the measure last year with bipartisan support, and Democratic President Joe Biden signed it into law. It requires TikTok's China-based parent company ByteDance to sell the platform or face a U.S. ban on Jan. 19. 

The dispute goes before the top U.S. judicial body at a time of growing trade tensions between the world's two biggest economies and just 10 days before Trump is due to begin his second term as president.

The Justice Department, defending the law, has said TikTok poses a threat to U.S. national security because of its access to immense amounts of data on American users, from locations to private messages, and its ability to secretly manipulate content that they view on the app. 

TikTok and ByteDance rebut the national security claims, instead portraying the law as running afoul of the First Amendment. If the law is allowed to stand "then Congress will have free rein to ban any American from speaking simply by identifying some risk that the speech is influenced by a foreign entity," they told the Supreme Court in a filing. 

Trump has said he has a "warm spot" for TikTok and has vowed to "save" a platform on which his campaign generated "billions of views." 

"President Trump opposes banning TikTok in the United States at this juncture, and seeks the ability to resolve the issues at hand through political means once he takes office," Trump's lawyer John Sauer wrote in a filing, asking the justices to put the law on hold. 

Sauer is Trump's pick to serve as U.S. solicitor general, the government's chief lawyer at the Supreme Court. 

STATE ATTORNEYS GENERAL WEIGH IN

By contrast, many Republican lawmakers and officials are pressing the court - whose conservative majority includes three justices appointed by Trump during his first term as president - to back the Biden administration in its defense of the measure. 

Republican attorneys general from 22 states filed a brief with the court disagreeing with TikTok's arguments and asking the justices to uphold the statute.

"Allowing TikTok to operate in the United States without severing its ties to the Chinese Communist Party exposes Americans to the risk of the Chinese Communist Party accessing and exploiting their data," these state officials, led by Montana Attorney General Austin Knudsen, wrote in their filing.

Montana tried to ban TikTok at the state level but was blocked by a federal court.

Former Senate Republican leader Mitch McConnell has compared TikTok's litigation to a hardened criminal seeking a "stay of execution." The Republican chairman and the top Democratic member of a U.S. House of Representatives panel focused on China issues urged the justices to uphold the measure to "protect the American people from foreign threats."  

Biden's administration on Jan. 3 asked the justices to reject Trump's request to put the ban on hold.

Trump's support for TikTok is a reversal from 2020, when during his first term as president he tried to block the app and force its sale to American companies. Trump has since said a TikTok ban would benefit Meta-owned platforms Facebook and Instagram, which he has criticized for suspending him after the attack on the U.S. Capitol by his supporters on Jan. 6, 2021. 

TikTok, ByteDance and some users who post content on the app appealed a Dec. 6 ruling by the U.S. Court of Appeals for the District of Columbia Circuit upholding the law. 

If the Supreme Court upholds the statute, Edgar said, "the stakes for internet freedom both in the United States and around the world are high." 

The U.S. government, Edgar added, "will be on solid ground if it chooses to regulate or ban any digital platform with substantial involvement from foreign investors." Another widely used platform, Telegram, "may be next," Edgar added.

In a Dec. 13 letter, U.S. lawmakers told Apple and Alphabet's Google, which operate the two main mobile app stores, that they must be ready to remove TikTok from those stores on Jan. 19. 

While U.S. users probably will still be able to use TikTok after the deadline because it is already downloaded on their phones, according to experts, over time the app will become unusable without software and security updates.