Four top House Democrats are urging the Pentagon to reject a request from the Department of Homeland Security to use $450 million to enhance an existing segment of the U.S.–Mexico border wall and to build new border infrastructure at a military range in Arizona.

“In countless hearings and briefings this year we have heard from senior civilian and military leadership on the readiness challenges our military currently faces,” the lawmakers wrote in a letter to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE on Wednesday that was publicly released Thursday.

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“We have heard about the Department of Defense's plans to rebuild military readiness, to include investing in additional spare parts and maintenance, increasing training opportunities, growing certain military occupational specialties, and modernizing aging weapons systems," they continued. "We have also heard the repeated calls from the Department for stable and predictable funding. With that in mind, we fail to see how diverting $450 million away from efforts to rebuild military readiness is in the department or the taxpayers’ best interests.”

The lawmakers argued that it would be inappropriate to use funds on a border wall at a time when 31 percent of Pentagon facilities are in "poor or failing condition" and the department has a facilities maintenance backlog totaling $116 billion.

“Funding the requested border infrastructure project may divert resources away from authorized and appropriated military construction projects, deferring critical investments to meet new mission requirements or replacing failing infrastructure,” they wrote.

The letter, signed by Reps. Adam Smith David (Adam) Adam SmithWhen 'Buy American' and common sense collide Overnight Defense: Marine Corps brushes off criticism of Marines' appearance in GOP convention video | US troops injured in collision with Russian vehicle in Syria | Dems ask for probe of Vindman retaliation allegations Democrats press Pentagon watchdog to probe allegations of retaliation against Vindman brothers MORE (D-Wash.), ranking member of the House Armed Services Committee; Nita Lowey Nita Sue LoweyTop House Democrats call for watchdog probe into Pompeo's Jerusalem speech With Biden, advocates sense momentum for lifting abortion funding ban Progressives look to flex their muscle in next Congress after primary wins MORE (D-N.Y.), ranking member of the House Appropriations Committee; Peter Visclosky Peter John ViscloskyBottom line Key races to watch in Tuesday's primaries Trump's emergency declaration looms over Pentagon funding fight MORE (D-Ind.), ranking member of the House Appropriations Defense Subcommittee; and Debbie Wasserman Schultz Deborah (Debbie) Wasserman SchultzFlorida Democrat introduces bill to recognize Puerto Rico statehood referendum Five things to watch at the Democratic National Convention Michelle Obama wishes Barack a happy birthday: 'My favorite guy' MORE (D-Fla.), ranking member of the House Appropriations military construction subcommittee, also raised concerns that Congress did not authorize or appropriate military construction funds for a wall.

“We note that military construction projects that exceed the minor construction threshold require line-item authorization and appropriation by Congress,” they wrote. “Congress has not provided an authorization or appropriation for a border infrastructure military construction project and we do not believe the scope or justification for the project warrants the use of emergency construction authorities or consideration outside of the annual authorization and appropriations process.”

The lawmakers questioned the prioritization of such funding for the border wall segment and asked Mattis about what "statutory authority" would be used to finance the infrastructure.

Instead of putting the $450 million toward the 31 miles of border infrastructure, the lawmakers wrote, "the Department of Defense should focus its time and resources on infrastructure needs that address the military readiness shortfalls that have been repeatedly highlighted by civilian and military leadership."

President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE first raised the possibility of using Pentagon funding for the wall in March, amid frustration that Congress had not yet funded his proposed wall along the southern border.

Shortly afterward, a Pentagon spokeswoman confirmed that the department was looking at the Barry M. Goldwater Air Force Range as a possible place to build a stretch of wall. The land is a bombing range near the U.S.–Mexico border in Wellton, Ariz.

In their letter, the Democrats said the Department of Homeland Security sent the Pentagon a request last month “to enhance existing border fencing and construct new border infrastructure” along 31 miles of the range.

“With an estimated cost of $450 million, we write to express our strong opposition to the use of Department of Defense funds for this purpose and respectfully urge you to deny the Department of Homeland Security’s request,” they wrote.