Updated August 2, 2021 at 6:34 PM ET

Immigration advocates who had been negotiating with the Biden administration to end a Trump-era rule that blocks most migrants from entering the United States have given up waiting.

The American Civil Liberties Union and other groups are going back to court. They plan to file a preliminary injunction to stop the continued use of the Title 42 public health law that has allowed border agents to swiftly remove tens of thousands of migrants and asylum-seekers arriving at the southern border.

"The Biden administration has left us no choice but to go back to court. It's been seven months, and Title 42 is still in place," Lee Gelernt, a lawyer for the ACLU, told NPR. "We believe this is our only option."

The Biden administration has kept the policy in place, citing concerns about the pandemic, including the explosion of cases of the contagious delta variant of the coronavirus. The new wave of cases also led the administration to extend restrictions on international tourists from many countries and impose new vaccine requirements for federal workers.

White House press secretary Jen Psaki said Monday that the administration is deferring to the Centers for Disease Control and Prevention on whether the policy is necessary.

"The president views it as a public health measure where the CDC is going to continue to provide guidance on how long it needs to be in place," Psaki said in Monday's briefing. "We have not given a timeline of when ... they will lift Title 42, but we will look for them to provide us that guidance."

CDC says migrants pose risk of spreading the virus

The CDC issued a statement a few hours later saying it would keep the order in place for now and review it every 60 days. It cited the variants and risks of COVID-19 spreading in places such as border stations.

"Introduction of such non-citizens, regardless of their country of origin, migrating through Canada and Mexico into the United States creates a serious danger of the introduction of COVID-19 into the United States," the CDC statement said. "The danger is so increased by the introduction of such non-citizens that a temporary suspension is necessary to protect the public health."

But the advocates for migrants — including the Texas Civil Rights Project, RAICES, the Center for Gender and Refugee Studies and Oxfam — argue the rule is being misused to illegally block vulnerable people from seeking humanitarian protection in the United States.

Spokespersons for the Justice and Homeland Security departments did not immediately respond to requests for comment.

Advocacy groups say Title 42 is being used as an excuse

Groups that have pushed to end the program have long argued the administration is using Title 42 less as a way to control the spread of the coronavirus — and more to curb migration and mitigate political pressure from Republicans.

The Trump administration first invoked Title 42, a section of the Public Health Safety Act, early in the pandemic, arguing it was justified to block entry of certain groups "in the interest of public health."

The groups sued the Trump administration over its use of the measure last year but put the case on hold when Biden took office, pending negotiations. Those talks are at an impasse. Gelernt said the groups now plan to seek an immediate preliminary injunction to stop the practice.

"The Biden administration asked for some time to fix what it said were problems created by the prior administration. We gave them more than sufficient time," he said.

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Biden reversed other Trump measures but not this one

When Biden took office, he quickly reversed several of Trump's harshest immigration policies, pledging a more "humane" system. But the new administration kept using Title 42 to shut out most people seeking asylum, to the consternation of advocates who had sued the U.S. government to stop it. The administration made exceptions for unaccompanied children and some families.

The use of Title 42 gave the Biden administration additional time to grapple with the largest surge of migrants to the border in recent history. According to U.S. Customs and Border Protection, border officials encountered migrants 188,829 times in June, the highest single month total in years.

The Biden administration has pushed back on criticism that it would use Title 42 for any reason other than to protect the public health of Americans and U.S. residents. In March, Homeland Security Secretary Alejandro Mayorkas said: "We are using it as the Title 42 authority was intended and not as a bludgeoning tool under immigration law that the prior president used."

Some advocates had expected the Biden administration to begin phasing out the program as early as last month, but the administration has since begun taking more aggressive steps against the delta variant. Gelernt said that's not a good enough reason to keep the practice.

"We do not believe that the delta variant is a basis for expelling people without a hearing," he said. "The country is in a much better position than when we filed the lawsuit, given the availability of vaccines and testing."

Polls show concern over Biden's handling of the migration surge

Political opponents have used the surge in migration to attack Biden, and many Republicans see the issue as a potent weapon to use for the midterm elections next year. While Biden has high approval ratings for the way he has handled the pandemic, polls show concern about immigration.

Biden has repeatedly defended himself against criticism of the border surge by explaining that the vast number of migrants were being returned — a defense made possible largely because of Title 42.

"Thousands — tens of thousands of people who are — who are over 18 years of age and single — people, one at a time coming, have been sent back, sent home," Biden said during a March 25 news conference.

Out of the more than 900,000 encounters border officials have had with migrants between January and June, the public health measure has been used more than 575,000 times to return migrants back across the border. The Biden administration has made an exception for children traveling without their parents and some families.

Advocates for migrants have charged that the Biden administration's use of Title 42 was an extension of the Trump administration's efforts to deny asylum rights unfairly.

"There was a lot of damage done during the Trump administration," said Dylan Corbett, executive director of the Hope Border Institute. "We have to climb back from that. That means things like making sure that we are rebuilding our refugee system to receive refugees from around the world, that we are taking positive steps to receive asylum-seekers and restore the process of asylum at the border. But we've seen that politics can interfere with it."

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