In the summer of 2020, in full re-election mode and looking for new ways to punish China, President Donald J. Trump threatened to cut off TikTok from the phones of millions of Americans unless its parent company agreed to sell all of its U.S. operations to American owners. The effort collapsed.

Now, more than two years later, after lengthy studies of how Chinese authorities could use the app for everything from surveillance to information operations, the Biden administration is attempting a strikingly similar move. It is better organized, vetted by lawyers, and coordinated with new bills in Congress that appear to have considerable bipartisan support.

Yet making TikTok safe from Chinese exploitation — as a tool for Chinese officials to surveil Americans’ tastes and whereabouts, as an entry point into the phones that contain their whole lives and as a way to pump out disinformation — turns out to be harder than it looks.

The tensions over the app will come to a head on Thursday, when TikTok’s Singapore-based chief executive, Shou Chew, testifies before the House Energy and Commerce Committee, a hearing that will give Democrats and Republicans alike a rare chance to air their suspicions directly to the company. On Tuesday, Mr. Chew posted a TikTok from the company’s main account, declaring that “some politicians” are trying to take the app away from 150 million users in the United States, including small businesses.

But after two years of negotiations with TikTok about building in new protections, it is not clear there is anything the company can do, short of turning the entire operation over to Americans, that will satisfy the concerns of U.S. intelligence agencies. The Justice Department’s No. 2 official and others have effectively rejected proposals by TikTok’s corporate parent, ByteDance, to address the concerns.

Any decision to remove the app, either banning it for 150 million users in the United States or blocking further downloads, would be politically fraught for Mr. Biden. No one encapsulated the political dilemma more pithily than Gina Raimondo, the commerce secretary, who is at the center of new export controls imposed on high-technology goods to China.

“The politician in me thinks you’re going to literally lose every voter under 35, forever,” she said recently to Bloomberg News.

Ms. Raimondo and other officials quickly add that bad politics is no reason to back away from a total ban if the national security threat warrants it. The problem is made more complex by the fact that some of the world’s largest news organizations, including The New York Times, now have TikTok accounts, meaning that shutting down the app could appear to be shutting down the spread of fact-based news to counter Chinese disinformation.

“A lot of this is a game of chicken,” said James A. Lewis, who runs the cyberthreats program at the Center for Strategic and International Studies. But he believes Mr. Biden has a far greater chance of success than his predecessor did.

“Different from the Trump administration, I think this administration has a chance of winning — attitudes have changed toward China,” he said. Several new bills that would, in different ways, give explicit new authority to the president to shut down TikTok have received bipartisan support. They are propelled by the intelligence community’s conclusion, contained in the Worldwide Threat Assessment delivered to Congress, that China remains the “broadest, most active and persistent” cyberthreat to the country.

Yet so far, the threat from TikTok is largely theoretical.

There have been a handful of cases of abuse, including efforts to geolocate reporters who published leaked information about the company. But the administration has not presented comprehensive, declassified evidence of a systemic effort to use the app to advance the Chinese government’s collection efforts.

That has not stopped nearly 30 states from banning TikTok from official government or contractor phones, and federal employees are being made to remove it as well — though not from their personal devices.

There are three areas of clear concern. The first is where TikTok stores the data of its United States users. Until recently, much of it was on ByteDance-run servers in Singapore and Virginia, which many feared would allow China to require TikTok to turn over user data under Beijing’s national security laws. This year TikTok tried to pre-empt this argument, saying it would delete the data of its American users from the ByteDance servers and move them to servers run by Oracle, an American cloud computing firm.

Then comes the harder question — who writes the algorithm, the code that is TikTok’s secret sauce. That code assesses a user’s choices and uses them to select more material to feed the user — a favorite dance routine, or maybe an interesting news story. The algorithms have been written in China, by Chinese engineers who have refined the art of giving users what they want to see. The worry, Matt Perault and Samm Sacks wrote recently on the Lawfare blog, is that “TikTok could unilaterally decide to prioritize content that would threaten or destabilize the United States.” Again, it hasn’t happened yet, at least not through TikTok.

And finally, there is the issue of whether an app whose algorithm few understand could be a gateway for outsiders, including the Chinese ministry of state security, to get into the phones of Americans — to find out not their dance preferences, but the vast trove of data they carry around in their hip pockets.

In November, Christopher A. Wray, the F.B.I.’s director, warned that the Chinese government could use TikTok’s algorithm for “influence operations.” Gen. Paul M. Nakasone, the head of U.S. Cyber Command and the director of the National Security Agency, echoed those concerns this month, saying that “it’s not only the fact that you can influence something, but you can also turn off the message as well when you have such a large population of listeners.”

TikTok has sought to respond to misinformation concerns with a lengthy list of updated policies for moderating videos, including new restrictions and labeling rules for deepfakes — highly realistic fake videos made with artificial intelligence. TikTok, for example, will not allow deepfakes of private figures and will bar those of public figures if the content is used for endorsements. It also offered more detail on how it will “protect civic and election integrity.”

A spokeswoman for TikTok did not respond to a request for comment.

The fight over the app had already become a knotty legal issue by the time Mr. Biden inherited it from Mr. Trump in 2021.

Federal courts had ruled that Mr. Trump did not have the power to execute his proposed ban of the app from Apple’s and Google’s app stores, taking away crucial leverage the White House had used to get ByteDance to consider selling TikTok.

Mr. Biden issued an executive order in June 2021 rolling back Mr. Trump’s threat of a ban. He left in place the order that demanded ByteDance divest the app. But staff members for a group of federal agencies that vet foreign companies in America, the Committee on Foreign Investment in the United States, were considering a third option: negotiating an agreement with TikTok that would resolve the national security concerns but stop short of forcing ByteDance to sell the app.

Under its latest proposal, TikTok would not only store U.S. user data on Oracle servers in the United States; the cloud computing company would also monitor its content recommendation algorithm — which TikTok says is a hedge against the app being used to spread propaganda. And the entity governing the app in the United States would be overseen by a board of three people approved by the government.

But that proposal didn’t satisfy hawks in Washington. Some in the administration — including Lisa O. Monaco, the deputy attorney general — had concerns its terms weren’t strict enough. The administration also faced growing pressure from lawmakers who said the app should be banned entirely.

Now, the Biden administration is pursuing a new strategy.

Publicly, it backed legislation earlier this month from a bipartisan group of senators that would give the Commerce Department clearer power to ban the app, potentially restoring the government’s leverage over ByteDance. Privately, administration officials told TikTok they wanted its Chinese ownership to sell the app or face a possible ban. Should the legislation pass, it would significantly strengthen the administration’s hand in forcing a sale.

Peter Harrell, a lawyer and former senior director for international economics and competitiveness on the National Security Council, said the proposed legislation is “important because as the U.S. deals with TikTok and other Chinese apps it needs some clear-cut legal authorities to regulate and compel actions” that don’t exist in current law.

A White House spokeswoman declined to comment beyond pointing to its existing support for the legislation.

At moments, TikTok has undercut its own arguments. It has said it would not turn over information about its users to the Chinese government — though China’s national security law would clearly require it to do just that if the nation’s intelligence services ordered its Chinese employees to do so.

When Forbes reported in October that a China-based team at ByteDance planned to use TikTok to monitor the locations of some Americans, TikTok’s communications team responded on Twitter that the publication’s work lacked “both rigor and journalistic integrity.” It also said TikTok had “never been used to ‘target’” U.S. politicians or journalists.

Two months later, ByteDance admitted that four of its employees, including two based in China, had gained access to the IP addresses and other data of two reporters, as well as some people connected to the reporters through their TikTok accounts. The employees were trying to determine if the individuals had been meeting with ByteDance employees, so they could attempt to discern the source of leaks to the journalists.

TikTok dismissed the case as an anomaly, and fired the employees. It said it set up systems to prevent a recurrence. And without question, American companies have had similar insider incidents of privacy breaches.

But in the current atmosphere in Washington, especially after the downing of a Chinese surveillance balloon that crossed the United States in January, any evidence of Chinese surveillance feeds a deep, bipartisan desire to crack down on China’s entry points to American networks. And of those, there is no bigger one — or more influential — than TikTok, which is why the path the administration takes over the next few months may set a precedent that goes far beyond TikTok’s immediate fate.

Julian Barnes contributed reporting from Washington.