By ELLIOT SPAGAT (Associated Press)

SAN DIEGO (AP) — President Joe Biden on Tuesday ordered a halt to asylum processing at the U.S. border with Mexico when illegal entries reach a threshold deemed excessive.

The measure takes effect immediately because the new policy is triggered when arrests for illegal entry reach 2,500. About 4,000 people already are entering the U.S. each day. It was a major policy shift on a critical election-year issue that’s exposed Biden to Republican criticism over an unprecedented surge in new arrivals in an election year.

The measure

Advocates say the new measure will put migrants in danger and violate international obligations to provide safe haven to people whose lives are threatened. The Biden administration denies that.

Legal challenges are imminent.

There are also serious questions of whether the new measure can stop large-scale migrant entries. Mexico has agreed to take back migrants who are not Mexican, but only in limited numbers. And the Biden administration doesn’t have the money and diplomatic support it needs to deport migrants long distances, to China and countries in Africa, for example.

Many who claim asylum today are free to live and work in the United States while their claims slowly wind through overwhelmed immigration courts.

Some questions and answers about Biden’s presidential proclamation:

HOW WILL THIS PLAY OUT ON THE GROUND?

The threshold triggers a halt on asylum until average daily arrests for illegal crossings fall below 1,500 for a week straight. The last time crossings were that low was around the peak of the COVID-19 pandemic, in July 2020.

The pandemic-related asylum restrictions known as Title 42 carried no legal consequences and encouraged repeat attempts. Now, migrants will be issued deportation orders even if they are denied a chance to seek asylum. That will expose them to criminal prosecution if they try again and ban them for several years from legally entering the country. It’s a key difference.

Migrants who express fear for their safety if they’re deported will be screened by U.S. asylum officers but under a higher standard than what’s currently in place. If they pass, they can remain to pursue other forms of humanitarian protection, including those laid out in the U.N. Convention Against Torture.

Unaccompanied children are exempt, raising the possibility that some parents may send their sons and daughters across the border without them.

WHAT ROLE DOES MEXICO PLAY?

A critical one.

The U.S. has limited funding to fly people home to more than 100 countries, including many in Africa and Asia. It also lacks diplomatic sway and logistical arrangements to deport large numbers to many countries, including China, Russia and Venezuela.

A 1997 court order generally limits detention of families with a child under 18 to 20 days, a highly ambitious and perhaps unrealistic turnaround time to screen people who express fear of deportation and then put them on a flight.

Even for single adults, U.S. Immigration and Customs Enforcement has enough funds to only detain about 34,000 people at a time.

Mexico has agreed to take back up to 30,000 people a month from Cuba, Haiti, Nicaragua and Venezuela, in addition to Mexicans. Its commitment does not extend to other nationalities.

This year, Mexico has also made it far more difficult for migrants to reach the U.S. border, largely by preventing them from riding freight trains and stopping them on buses to turn them around to southern Mexico. While Mexican authorities are blocking migrants’ advance, relatively few are deported, causing many to be stuck in Mexican cities far from the U.S. border.

Alicia Bárcena, Mexico’s foreign relations secretary, told reporters last month that Mexico won’t allow more than 4,000 illegal entries a day. President-elect Claudia Sheinbaum, who takes office Oct. 1, is expected to continue policies of her mentor and Mexico’s current president, Andrés Manuel López Obrador.

HAS THIS BEEN TRIED BEFORE?

This is the latest in a series of measures under the Biden and Trump administrations to deter asylum-seekers, none of which have had lasting impact.

In May 2023, Biden imposed similar obstacles to asylum for anyone who crossed the border illegally after passing through another country, such as Mexico. A federal appeals court allowed those restrictions to stay in place while advocates challenge it, but it appears to have little impact.

Illegal crossings fell after last year’s restrictions took effect, but the lull was short-lived as the number of screening officers was inadequate for the enormous task. The rule’s application in only a small percentage of arrests showed how budgets can fail to match ambitions.

Biden invoked a section of the Immigration and Nationality Act that allows the president to ban entry for groups of people if their presence “would be detrimental to the interests of the United States.” President Donald Trump used these powers to ban entry of people from some predominantly Muslim countries, though advocacy groups are expected to argue that Biden failed to meet that “detrimental” criterion.