A new border crackdown unveiled by the Biden administration last week is likely to disproportionately affect families, whose soaring numbers in the last decade have drastically changed the profile of the population crossing the southern border.

Family units have come to represent a substantial share of border crossers, accounting for about 40% of all migrants who have entered the United States this year. Families generally have been released into the country quickly because of legal constraints that prevent children from being detained for extended periods.

They then join the millions of undocumented people who stay in the United States indefinitely, under the radar of U.S. authorities, as they wait for court dates years in the future.

But according to a memo issued by the Homeland Security Department and obtained by The New York Times, families will be returned to their home countries within days under President Joe Biden’s new border policy, which temporarily closed the U.S.-Mexico border to most asylum-seekers Wednesday.

The implications of the new policy are enormous for families, who are some of the most vulnerable groups making the journey to the United States. Advocates warn that it could have dangerous repercussions, making parents more likely to separate from their children or send them alone to the border because unaccompanied minors are exempt from the new policy.

The vast majority of families seeking asylum are from Central America and Mexico, which places them in a category described in the memo as “easily removable,” akin to single adults from those regions. The memo lays out how authorities are to carry out the new policy.

Smuggling organizations had long used the likelihood that migrants would be released after entering the country unlawfully as a selling point. But the new border policy makes no distinction between how families and single adults who illegally enter the country are handled, erasing the perceived advantage of arriving as a family.

Instead, families would be prioritized for expedited removal, a Biden administration official said, asking for anonymity to discuss the executive action.

“This seems to be a remarkably cynical strategy to run up the numbers of deportees by targeting the most vulnerable segment of the migration flow,” said Wayne Cornelius, director emeritus of the Mexican Migration Field Research Program at the University of California, San Diego.

But with the number of people crossing the border at record levels, the new policy was an attempt to cut down on illegal immigration and ameliorate one of Biden’s biggest vulnerabilities in his campaign against former President Donald Trump. Biden is under pressure, even from within his own party, to do something about immigration.

In a significant change that mimics a Trump-era practice, some families who argue that they should be an exception to the new asylum restrictions will have a so-called credible-fear interview during detention at the border, which is difficult to pass while in custody and without a lawyer.

“It is horrifying to hear that the Biden administration is quietly rolling out one of the absolutely worst Trump border pilot programs — subjecting families to rapid credible-fear interviews while detained in Border Patrol custody,” said Taylor Levy, an immigration lawyer.

The removal of families is made easier by the fact that most are from Guatemala, Honduras and other countries in the Western Hemisphere. Those countries are relatively close to the United States and already accept repatriations, unlike many countries in Africa and Asia, which are far away and whose governments are less likely to accept deportees.

Biden’s order, which went into effect Wednesday, empowers border agents to turn back — or swiftly deport — migrants who come into the country illegally, with few exceptions.

The border will reopen only when the number of unauthorized crossings drops to fewer than 1,500 for seven days in a row and stays that way for two weeks. The numbers have not been that low in years; in December, there were about 10,000 illegal crossings every day. More recently, the figures have hovered around 3,000 crossings per day.

In recent years, Mexican families displaced by cartels that control swaths of territory have been crossing the border in ever-greater numbers to seek safety in the United States.

In the first eight months of the 2024 fiscal year, which started Oct. 1, the Border Patrol apprehended nearly 150,000 Mexican migrant families entering the United States illegally, compared with 87,014 in 2023 and 17,040 in 2020.

“Huge numbers of Mexican families have been coming, and they are easy to send back,” said Kathleen Bush-Joseph, a policy analyst at the Migration Policy Institute, a nonpartisan think tank, because they can be returned to their country on a bus.

The removal of families, and the exemption for unaccompanied minors, under the new restrictions are almost certain to lead to family separations as desperate parents decide to send their children alone, often with smugglers, she said.

In May last year, a 4-year-old child was dropped into the United States over the steel wall that separates San Diego from the Mexican city of Tijuana. The child survived. Two years earlier, agents rescued two young sisters, 3 and 5, who had been dropped on the barrier’s U.S. side in New Mexico.