Portside

President-elect Donald J. Trump is likely to justify his plans to seal off the border with Mexico by citing a public health emergency from immigrants bringing disease into the United States.

Now he just has to find one.

Mr. Trump last invoked public health restrictions, known as Title 42, in the early days of the pandemic in 2020, when the coronavirus was tearing across the globe. As he prepares to enter office again, Mr. Trump has no such public health disaster to point to.

Still, his advisers have spent recent months trying to find the right disease to build their case, according to four people familiar with the discussions. They have looked at tuberculosis and other respiratory diseases as options and have asked allies inside the Border Patrol for examples of illnesses that are being detected among migrants.

They also have considered trying to rationalize Title 42 by arguing broadly that migrants at the border come from various countries and may carry unfamiliar disease — an assertion that echoes a racist notion with a long history in the United States that minorities transmit infections. Mr. Trump’s team did not respond to a request for comment.

The plan to invoke the border restrictions based on sporadic cases of illness or even a vague fear of illness — rather than a major disease outbreak or pandemic — would amount to a radical use of the public health measure in pursuit of an immigration crackdown. Even when the coronavirus was spreading, the use of the health authority to turn away migrants prompted scrutiny from the courts and public health officials.

But Mr. Trump’s immigration advisers, led by Stephen Miller, his pick to be deputy chief of staff, believe they are entering a political environment that will welcome more aggressive border enforcement, particularly after some Democrats embraced using restrictions like Title 42, according to people familiar with the planning. President Biden used it to turn away thousands of migrants before eventually deciding to lift it, well after his public health advisers said the restrictions were no longer useful for the purpose of stopping the spread of disease.

Title 42, which is part of the Public Service Act of 1944, grants power to health authorities to block people from entering the United States when it is necessary to avert a “serious danger” posed by the presence of a communicable disease in foreign countries.

Mr. Miller has long considered Title 42 a key tool for his goal of shuttering the border to migration. He has essentially been on a yearslong quest to find enough examples of diseases among migrants to justify the use of the law.

Even before the spread of the coronavirus, Mr. Miller asked aides to keep tabs on American communities that welcomed migrants to see if diseases broke out there. He seized on an outbreak of mumps in immigration detention facilities in 2019 to push for using the public health law to seal the border. He was talked down in most of the cases by cabinet secretaries and lawyers — until the advent of the coronavirus.

The Centers for Disease Control and Prevention, not the White House, is responsible for assessing whether the public health rule is necessary at the border. And even when the pandemic spread throughout the United States, C.D.C. officials pushed back on the Trump White House’s position that turning away migrants was an effective way to prevent the spread of diseases.

Martin Cetron, the director of the agency’s Division of Global Migration and Quarantine, told a House committee that the implementation of the border restrictions “came from outside the C.D.C. subject matter experts” and was “handed to us” by the White House.

When Mr. Biden came into office, he initially kept the public health rule in place at the border, even when C.D.C. officials told his top aides there was no clear public health rationale for keeping the border shut to asylum seekers. Both the Biden and Trump administrations argued the rule was needed to prevent the spread of diseases in detention facilities at the border. But Mr. Biden’s top White House aides were privately concerned that lifting the rule would lead to a surge in migration.

During his second stint in the White House, Mr. Trump’s team will focus on avoiding such pushback. He is intent on installing loyalists throughout his administration who are unlikely to try to stop his more aggressive proposals.