Republicans and immigration hawks on Thursday reacted gloomily to the Supreme Court ruling that the Biden administration can terminate the Trump-era "Remain-in-Mexico" policy -- fearing that the ongoing crisis at the southern border will only worsen as a result.

"I am disappointed in SCOTUS allowing Biden to dissolve the Remain-in-Mexico program, one of our last & best protections against the Dems’ border crisis," Texas Attorney General Ken Paxton said in response to the ruling.

SUPREME COURT HANDS BIDEN VICTORY, ALLOWS END TO ‘REMAIN IN MEXICO’ POLICY

The Supreme Court, in its 5-4 ruling, said that the Department of Homeland Security could repeal the policy -- officially known as the Migrant Protection Protocols. It had previously been blocked from doing so by a district court in response to a lawsuit by Texas and Missouri which alleged that the Biden administration ended it unlawfully.

MPP was put in place in 2019 and expanded during that year’s migrant crisis. The policy allowed for migrants to be sent back to Mexico for their immigration hearings rather than be released into the United States. It led to tent courts being set up at various spots along the border, which migrants would be allowed to enter for their hearings.

The Trump administration said the policy was a central cog in its efforts to end "catch-and-release," which officials said was a key "pull factor" drawing migrants north. The claim was that if the U.S. stops releasing migrants into the U.S., fewer will make the journey.

Supreme Court justices

Seated from left are Associate Justice Samuel Alito, Associate Justice Clarence Thomas, Chief Justice John Roberts, Associate Justice Stephen Breyer and Associate Justice Sonia Sotomayor, Standing from left are Associate Justice Brett Kavanaugh, Associate Justice Elena Kagan, Associate Justice Neil Gorsuch and Associate Justice Amy Coney Barrett.  (Erin Schaff/The New York Times via AP, Pool, File)

However, migrant rights groups claimed the policy was cruel and inhumane, forcing migrants into camps on the Mexican side of the border which were often unsanitary and allegedly put migrants at risk of kidnappings and violence from gangs and cartels. They also pointed to the low success rate for migrants claiming asylum as evidence that the policy was stacked against asylum seekers.

President Biden pledged to end the policy and issued an order to do so shortly after entering office. The policy was scrapped and thousands of migrants enrolled into the program were allowed into the U.S. The ending of the policy coincided with a massive surge in migrants to the border, which has continued to the present day -- more than 239,000 migrants were encountered at the border in May alone.

BIDEN ADMIN EXPANDS TRUMP-ERA ‘REMAIN IN MEXICO’ TO RIO GRANDE VALLEY SECTOR, CITING COURT ORDER

However, in response to the lawsuit from Texas and Missouri, the administration was found to have ended the policy unlawfully and was forced to reinstate it until the case was resolved. However, only a few thousand migrants are currently enrolled in the program -- with most migrants being either released into the U.S. or expelled under the Title 42 public health order.

At issue was whether the administration violated federal law -- which plaintiffs said mandated the detention of inadmissible immigrants. 

The court found in favor of the administration, and said the lower court should now look for the first time at whether the Biden administration's Oct. 29, 2021 memo terminating the policy was in compliance with Administrative Procedure Act protocols that govern the actions of federal agencies.

Republicans have secured a number of legal victories over the Biden administration on immigration, but this marked a major defeat in those efforts. Republican lawmakers expressed fear that the raging crisis at the border would now only continue now that the Biden administration could unravel such a key policy.

"Today’s Supreme Court ruling is very disappointing," Rep. John Katko, Ranking Republican on the House Homeland Security Committee, said in a statement." But to be clear, we are in this position because of the utter disregard President Biden has for defending our borders and our homeland security. President Biden would rather fight this case all the way to the U.S. Supreme Court than secure the border. The ‘Remain in Mexico’ policy was a critical tool for Border Patrol Agents to manage the influx of migrants flooding into our country."

BIDEN ADMIN ENROLLED FEWER THAN 300 MIGRANTS IN ‘REMAIN IN MEXICO’ IN DECEMBER

Katko said that the administration has " no plan to address this nightmare at the border, apart from releasing migrants into communities across the country."

"Nothing short of a fully-reimplemented and enforced MPP in conjunction with Title 42, physical barriers, and more resources for Border Patrol can fix the border crisis at this point," he said. "When Republicans take back the majority in the House next Congress, we fully intend to see these important policies reinstated."

Photo of Joe Biden, Greg Abbott

Texas Republican Gov. Greg Abbott, on right, said the Supreme Court ruling giving the Biden administration power to terminate the Trump-era "Remain-in-Mexico" policy will only "embolden the Biden Administration’s open border policies." (Photo by Chip Somodevilla/Getty Images  |  USA TODAY NETWORK via Reuters Connect   )

Texas Gov. Greg Abbott said that the decision "will only embolden the Biden Administration’s open border policies."

He pointed to recent migrant deaths at the border, including 53 who died in a tractor-trailer in San Antonio.

"Reinstating and fully enforcing Remain-in-Mexico would deter thousands more migrants from making that deadly trek, and President Biden should take that simple step to secure the border because it is the only humane thing to do," he said.

Rep. Michael Burgess, R-Texas, said ending the policy will " further exacerbate the humanitarian crisis at our southern border by giving people who cross the border illegally a free pass to stay in the country and never show up to their court date."

Meanwhile, the Federation for American Immigration Reform (FAIR) said that the decision "perpetuates this status quo and will allow the government to continue releasing inadmissible aliens into the United States, in direct violation of federal law."

"Today’s Supreme Court ruling highlights the need for Congressional action to mandate MPP and secure our southern border," the group, which calls for lower levels of immigration, said. "Courts cannot be relied on to reign in the Biden administration’s relentless flouting of immigration laws. Congress must step forward with needed reforms to stop this historic border crisis."

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The Center for Immigration Studies (CIS), meanwhile, noted that the issue of whether or not the program was ended unlawfully, has not yet been decided -- and also said courts must decide whether the administration is violating the law by releasing migrants into the interior en masse. In a statement, it said that the issues were of great importance "given the unprecedented growth in the illegal population under the Biden administration."

"Census data suggests the illegal alien population grew by 1.4 million since Inauguration Day 2021, while administrative data suggests the growth may be as large as two million," the group said in a statement. "Ending Remain in Mexico, as well as the Title 42 order, will only supercharge the flow of illegal immigration across the southern border (not to mention visa overstayers.)"

Fox News' Ronn Blitzer contributed to this report.