EXCLUSIVE: Sen. Bill Hagerty, R-Tenn., on Wednesday wrote to President Biden over his administration’s rollback of the Migrant Protection Protocols (MPP), warning that it could lead to tens of thousands of migrants flowing into the country and endanger U.S. communities.

In a letter to Biden, Hagerty expresses "serious concern" about reports that MPP -- known as Remain-in-Mexico -- is being ended by the administration, and warns that such a move could be particularly damaging when combined with other immigration policies.

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"Terminating MPP would unilaterally discard a significant diplomatic achievement, reward abuse of our generous asylum laws, and endanger American communities as a result," he said in the letter, obtained by Fox News.

"Further, it would exacerbate the damage caused by your other unilateral actions on immigration, including: barring virtually all deportations of illegal immigrants, including violent criminals; halting ongoing construction of a border wall demanded by law enforcement personnel on the front lines; and opening the door to admitting immigrants who cite the need to flee climate change," the senator said.

The Trump administration set up and expanded MPP during the 2019 migrant crisis as part of a broader agreement with Mexico, and hailed it as a key plank in its efforts to end "catch-and-release" -- by which migrants were released into the interior of the U.S.

Instead, MPP keeps migrants in Mexico as they await their immigration hearings. Proponents say the policy ended a key pull factor that brought migrants north, while critics say it is cruel and puts migrants in danger by leaving them in Mexico.

Biden promised to end the program and ordered a review of the program. Last week DHS announced it would begin to process migrants placed in the program -- estimating that there were 25,000 individuals enrolled in the protocols who still have active cases.  

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"As President Biden has made clear, the U.S. government is committed to rebuilding a safe, orderly, and humane immigration system," Secretary of Homeland Security Alejandro Mayorkas said in a statement. "This latest action is another step in our commitment to reform immigration policies that do not align with our nation’s values." 

That move, along with other immigration moves such as stopping wall construction and seeking to impose a 100-day pause on moratoriums -- has horrified Republicans, who are warning of a new border crisis.

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In his letter, Hagerty hailed the Trump administration’s MPP agreement with Mexico as an "unprecedented and highly successful partnership" and cited stats that Mexico apprehended 145,682 migrants. He too warned of a new border crisis if the policy was ended, and noted estimates that as many as 30,000 could be waiting south of the border.

"Once it becomes abundantly clear that the border is open, tens and hundreds of thousands more will follow. In effect, this will result in the mass importation of the equivalent of new towns and cities into the United States -- Cookeville, Tennessee, a mid-size city in my state, has roughly 30,000 residents, for example," he said.

Hagerty also noted the context of the COVID pandemic, "We need to focus on defeating the virus and getting Americans back to work and school, not on issuing an invitation to migrants to flood into our country unchecked."

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He asked Biden for answers to a number of questions -- including if migrants will be released into the general population, if they will be granted work permits, if they will be eligible to receive a vaccine, and if they will be removed if they are found to be ineligible for asylum. 

Biden is also expected in the coming days to unveil a new immigration bill that would grant a pathway to citizenship for millions of illegal immigrants -- although that bill will likely face stiff opposition from Republicans.