The Trump administration on Monday announced a sweeping new policy tightening restrictions for asylum seekers, in a move that could drastically reduce the number of Central American migrants eligible to enter the United States in this way.

The new rule, published in the Federal Register, would require most migrants entering through America’s southern border to first seek asylum in one of the countries they traversed – whether in Mexico, in Central America, or elsewhere on their journey. In most cases, only if that application is denied would they then be able to seek asylum in the United States.

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"Ultimately, today's action will reduce the overwhelming burdens on our domestic system caused by asylum-seekers failing to seek urgent protection in the first available country, economic migrants lacking a legitimate fear of persecution, and the transnational criminal organizations, traffickers, and smugglers exploiting our system for profits,” Homeland Security Acting Secretary Kevin McAleenan said in a statement, describing the "targeted changes" as critical.

Sure to ignite a new firestorm over the administration's immigration approach, the new policy follows the Trump administration's Migrant Protection Protocols, commonly referred to as the "remain in Mexico" policy. Under that policy, asylum seekers were often told to go back to Mexico to await hearings, rather than be allowed to remain in the U.S.

Democrats railed against that policy, with 2020 hopeful Beto O'Rourke calling it "inhumane."

The latest change is meant to crack down on asylum seekers coming to the U.S. more for economic reasons than to escape persecution in their home countries. The new policy does include a couple other exceptions, mainly for certain victims of human trafficking.

Attorney General Bill Barr said in a statement that the change would curb "forum shopping by economic migrants and those who seek to exploit our asylum system to obtain entry to the United States—while ensuring that no one is removed from the United States who is more likely than not to be tortured or persecuted on account of a protected ground.”

While Barr maintained the legality of the move, the new rule is poised to face an immediate court challenge. Shortly after the policy was announced, the American Civil Liberties Union vowed to sue.

“The Trump administration is trying to unilaterally reverse our country's legal and moral commitment to protect those fleeing danger. This new rule is patently unlawful and we will sue swiftly,” Lee Gelernt, deputy director of the group's Immigrants’ Rights Project, said in a statement.

U.S. law allows refugees to request asylum when they arrive at the U.S. regardless of how they did so, but there is an exception for those who have come through a country considered to be "safe." But the Immigration and Nationality Act, which governs asylum law, is vague on how a country is determined "safe"; it says "pursuant to a bilateral or multilateral agreement."

Right now, the U.S. has such an agreement, known as a "safe third country," only with Canada. Under a recent agreement with Mexico, Central American countries were considering a regional compact on the issue, but nothing has been decided. Guatemalan officials were expected in Washington on Monday, but apparently a meeting between Trump and Guatemalan President Jimmy Morales was canceled amid a court challenge in Guatemala over whether the country could strike such an agreement.

Trump administration officials say the changes are meant to close the gap between the initial asylum screening that most people pass and the final decision on asylum that most people do not win. The goal in part is to allow quicker determinations in these cases.

But immigrant rights groups, religious leaders and humanitarian groups have said the Republican administration's policies amount to a cruel effort to keep immigrants out of the country. Guatemala, Honduras and El Salvador are poor countries suffering from violence.

Along with the administration's recent effort to send asylum seekers back over the border, Trump has tried to deny asylum to anyone crossing the border illegally and restrict who can claim asylum.

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Reducing the number of asylum seekers, however, would ease the burden on federal agencies currently overwhelmed by the number of people looking to enter the United States. Children and adults crossing illegally are often separated, with the children placed in detention facilities that are lacking in resources, resulting in outcries against the administration’s current practices.

Acting Director of U.S. Citizenship and Immigration Services Ken Cuccinelli said on "Fox News Sunday" that action was needed to improve the situation. He said the current volume of people trying to enter the U.S. has resulted in poor conditions in detention facilities, and pointed a finger at lawmakers for not doing enough to fix the situation.

Cuccinelli recalled a visit to a facility in El Paso that had 950 people in it, despite a maximum capacity of 800.

"Once you're over those capacity points, you encounter problems," he said.

On Fox News' "Sunday Morning Futures," Sen. Lindsey Graham, R-S.C., cited the urgency for adopting new asylum laws.

Graham said that 30 percent of those who cross into the U.S. with children are "fake families," and that loopholes are used to let the same children come back over and over, bringing different adults with them.

The Associated Press contributed to this report.