EXCLUSIVE: More than 50 House Republicans on Tuesday warned President Biden of a brewing "crisis" of illegal migration at the southern border amid indications of a surge in numbers -- just as Biden has forged forward with a number of policies limiting border security and interior enforcement.

"This is not a political game -- we implore you not to let ideology blind you administration to the need to secure the border, to defend Americans, and to prevent another cartel-empowering humanitarian crisis," the letter, obtained by Fox News, says.

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The letter, from 51 Republicans and led by Rep. Chip Roy, R-Texas, warns of a "rising illegal migration crisis" and says sources on the ground have told them that Customs and Border Protection (CBP) agents have seen the average daily flow of migrants at the border surge to more than 3,500 a day from 2,000 last month. Meanwhile, the number of unaccompanied alien children is approaching 300 a day.

"CBP hasn’t seen numbers like this since the height of the immigration crisis in the summer of 2019," the lawmakers said. "CBP has already encountered over 200,000 illegal migrants since October and numbers are rising. For context, in the first three months of fiscal year 2020, CBP encountered 128,347," the lawmakers say.

The letter from Republicans, which includes Reps. Liz Cheney, R-Wyo., Andy Biggs, R-Ariz., Dan Crenshaw, R-Texas, Lauren Boebert, R-Colo., and Lance Gooden, R-Texas, also notes recent apprehensions of more than 11 Iranians across the Arizona border -- which has seen a broader increase in foot traffic too.

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The surge in migration comes amid a new policy shift toward illegal immigration by the Biden administration compared to the Trump administration. 

Biden has stopped construction of the wall at the southern border, proposed a pathway to citizenship for millions of illegal immigrants, strengthened protections for recipients of the Deferred Action for Childhood Arrivals (DACA) program and sought to impose a 100-day moratorium on deportations. He has also ordered a review of the Migrant Protection Protocols (MPP), which keeps migrants in Mexico as they await their immigration hearings.

Meanwhile, the administration recently opened a new overflow facility to deal with child migrants coming into the country.

The Republicans link the surge at the border to the new outlook by the new administration, arguing that numbers increased during the transition and that such an increase was "foreseeable."

"Further it is concerning to see your administration perceive our border security issues as a political game instead of the very serious threat open borders provide to Americans and the migrants seeking to come here," the letter says. 

"We are once again on the brink of a huge humanitarian crisis, endangering the lives of migrants and law enforcement officers at the hands of the violent cartels that your administration has further empowered," it says.

The lawmakers note that, so far, Biden has not rescinded the Title 42 public health order that allows migrants to be swiftly returned -- but warn that if he does, it would lead to "catastrophic" flows overnight.

The lawmakers request immediate briefings for House Republicans by new DHS Secretary Alejandro Mayorkas -- who was confirmed last week by the Senate.

The letter comes a day after White House press secretary Jen Psaki was pressed on reported guidance that would spare illegal immigrants with DUIs from deportation.

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"The priority for the enforcement of immigration laws will be on those who are posing a national security threat ... and on recent arrivals," Psaki said during a press briefing. "Nobody is saying that DUIs or assault are acceptable behavior, and those arrested for such activities should be tried and sentenced as appropriate by local law enforcement. But we’re talking about the prioritization of who is going to be deported from the country."

Fox News' Brittany de Lea contributed to this report.