FIRST ON FOX: Five Republican senators say they will withhold their support from any omnibus spending package due to what they say is a failure to adequately fund border security -- pointing specifically to a lack of funding for the wall at the southern border.

In a letter to fellow senators to be sent on Monday, Sens. Mike Braun, R-Ind., Marco Rubio, R-Fla., Mike Lee, R-Utah, Cynthia Lummis, R-Wyo., and Ted Cruz, R-Texas., say funding proposals released by the Senate Appropriations Committee "fail to provide America’s border agents with the proper security infrastructure necessary to defend America’s southern border from the continued crisis of unrestrained illegal migration."

The Trump-era border wall remains unfinished after the Biden administration put a stop to it. (Fox News)

The Trump-era border wall remains unfinished after the Biden administration put a stop to it. (Fox News)

"The failure to provide sufficient resources is a continuation of the Biden-Harris Administration’s inability and unwillingness to acknowledge the humanitarian and national security risks created by its open-border policies," they write in the letter exclusively obtained by Fox News. "We write to indicate that we will withhold support of any negotiated Fiscal Year 2022 omnibus funding agreement that fails to appropriately fund our nation’s border security."

The U.S. has been facing a dramatic and continuing crisis at the southern border, with more than 1.7 million migrant encounters in FY 2021, and more than 192,000 migrant encounters in September. 

Senator Mike Braun, a Republican from Indiana, walks to a Senate Republican caucus meeting at the U.S. Capitol in Washington, D.C., U.S., on Thursday, Oct. 7, 2021. Senate Majority Leader Chuck Schumer set up Thursday night votes on a short-term debt ceiling increase that would leave the battle to be rejoined less than two months from now, in the middle of an already packed congressional agenda. Photographer: Stefani Reynolds/Bloomberg via Getty Images

Senator Mike Braun, a Republican from Indiana, walks to a Senate Republican caucus meeting at the U.S. Capitol in Washington, D.C., U.S., on Thursday, Oct. 7, 2021. Senate Majority Leader Chuck Schumer set up Thursday night votes on a short-term debt ceiling increase that would leave the battle to be rejoined less than two months from now, in the middle of an already packed congressional agenda. Photographer: Stefani Reynolds/Bloomberg via Getty Images

While the Biden administration has blamed root causes like poverty and violence in Central America, Republicans have instead pointed to the rollback of Trump-era policies -- including President Biden’s decisions to halt border wall construction, of which more than 450 miles were built during the Trump administration.

The five Republicans note that, in addition to this, border wall funding was excluded from Democratic appropriation bills. The Homeland Security appropriation bill pulls $1.9 billion in wall funding and shifts it elsewhere. As a result, the bill assigns $14.5 billion in funding for Customs and Border Protection, down from more than half a billion dollars from the FY 2021 request -- even amid the crisis at the southern border.

"Rather than continuing to waste taxpayer dollars on border walls while ignoring proven, more effective, and less costly investments to improve border security, these funds will be refocused toward innovative and cost-effective capabilities and will also provide critical investments to support CBP employees," the bill summary states.

Republicans also noted that there is a $50 million earmark that would allow wall construction on federal land to be removed. They say this is despite "significant evidence" that wall construction is a "critical tool for America’s border security agents to combat illegal migration, drug smuggling, and human trafficking."

In particular, they point to Trump-era Department of Homeland Security (DHS) figures that show illegal entries dropped by as much as 87% in sectors where wall had been constructed.

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"The continuation of border security funding, particularly continued funding for physical barrier construction, remains necessary during the continued immigration crisis," they write. "As such, and in the defense of our nation, we will not offer support for any fiscal year 2022 omnibus agreement that omits this funding or authorizes the administration to remove previously constructed border security measures."

Republicans have continued to put pressure on the Biden administration over the wall. Just this week, Missouri and Texas requested a preliminary injunction to force continuation of the wall construction by the administration -- arguing that the redirection of funds assigned by Congress was unconstitutional.