The Pentagon estimates that the deployment of roughly 5,900 troops to the U.S.-Mexico border will cost $72 million, according to a Department of Defense (DOD) spokesman.

The total cost of the operation “has yet to be determined and will depend on the total size, duration, and scope of the DoD support to [the Department of Homeland Security],” Col. Rob Manning said in a statement late Tuesday, adding that the current cost estimate “to deploy, operate, sustain, and redeploy forces is approximately $72 million.”

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The estimate is based on the “current phased force laydown of approximately 5,900" active-duty service members staying at the southern border through Dec. 15, he said.

Manning added that the estimate “includes only those DoD forces and support requested and approved" as of Nov. 19.

Democratic lawmakers in the past week have upped the pressure on the Trump administration to provide answers on the troop deployment, which was ordered by President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE shortly before the midterm elections in November as a caravan of Central American migrants moved northward through Mexico.

Democrats and former uniformed leaders have accused the president of injecting partisan politics into the armed forces and not showing enough support for troops serving in combat.

A group of Democratic lawmakers on Tuesday sent 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 calling for a briefing and written justification from the military for the deployment.

The signatories, which included likely Democratic presidential candidates Sens. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.), Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.) and Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (D-Calif.), also urged Mattis to otherwise “curb the unprecedented escalation of DOD involvement in immigration enforcement.”