WASHINGTON — Democrats are poised to gain new oversight powers when they take control of the House in January, but a trio of notable party members are already grilling the Trump administration over its deployment of troops to the U.S.-Mexico border.

In a letter to Secretary of Defense Jim Mattis made public Tuesday, Sen. Elizabeth Warren, D-Mass., Rep. Jackie Speier, D-Calif., and Rep. Beto O’Rourke, D-Texas, asked more than 30 pointed questions about the planning behind the deployment, the cost of the operation, its timeline, and the rules of engagement for the operation — as well as whether the deployments will hurt military readiness overall.

The three are notable, given their prominence and roles on the defense committees. Warren serves on the Senate Armed Services Personnel Subcommittee and is seen as a potential 2020 contender; Speier is ranking member of the House Armed Services Military Personnel Subcommittee, and likely to take over that committee in the new Congress.

And while O’Rourke is a House Armed Services Committee Member who will be leaving Congress come January, his run for Senate has propelled him to a national stage, where he may wield influence ahead of 2020.

The Warren-led letter referenced a New York Times report that Pentagon officials privately view the deployment as a burden on morale and a waste of resources. Trump signed an order Oct. 26 for troops to provide civil authorities with an array of assistance, to include fixed and rotary wing aviation support; the building of temporary barriers, barricades, and fencing; medical team support, and temporary housing.

“We are distressed by this report, and increasingly concerned about the lack of planning for and continued lack of clarity surrounding this operation, the lack of a clear mission for the deployed troops, the cost of this operation, and the appearance that the President is using the military for partisan political purposes,” the lawmakers wrote.

“This is a disturbing report, and raises a number of questions as to whether or not the deployment of U.S. troops to the border by President Trump was a wise, effectively planned, and appropriate use of DOD resources,” the lawmakers wrote.

The lawmakers expressed concern with the unprecedented timing of the deployment, a week before a national election. They also noted that this was the largest deployment of active duty service members on such short notice since the earthquake in Haiti in 2010, and that active duty troops have not been sent to the border in over three decades.

Drawdown coming?

By coincidence, the letter was sent the same day Politico broke news that 5,800 troops who were rushed to the southwest border to aid domestic security agencies amid President Donald Trump’s pre-election warnings about a refugee caravan will start coming home as early as this week — just as some of those migrants are beginning to arrive.

That revelation came from the general overseeing the deployment, Army Lt. Gen. Jeffrey Buchanan, who leads the land forces of U.S. Northern Command and told Politico: “Our end date right now is 15 December, and I’ve got no indications from anybody that we’ll go beyond that."

The Pentagon has since pushed back on the idea that troop numbers would be drawing down, saying in a statement that the Army may “shift some forces to other areas of the border to engineering support missions in California and other areas," but that “no specific timeline for redeployment has been determined.”

Democrats and Republicans have criticized the administration’s border deployments, announced eight days before midterm elections, as a wasteful and politically motivated stunt. Trump had called a northward traveling caravan of several thousand Central American migrants an “invasion” — and asserted without supplying proof that the caravan contained “unknown Middle Easterners” and "criminals.”

In a statement late Monday, Rep. Adam Smith, the presumptive HASC chairman come January, blasted the president in the wake of the Politico report, saying the troops “should be returned to their regular duties.”

“The reports that President Trump is planning to withdraw some of the troops he sent to the border two weeks ago indicate just how empty, demagogic, and racially motivated this political stunt was. It appears to be an admission that there was no justification for the mission in the first place,” said Smith, D-Wash.

“It was not a respectful use of our military to take service members away from their duties and send them to the border as politicized props, and President Trump should not have done it. And there remains no rational explanation for the presence of those active duty troops who remain on the border,” Smith said.

Smith’s call to send troops home was joined by Sens. Brian Schatz, D-Hawaii, and Sen. Patty Murray, D-Wash., who sent a letter to Mattis on Monday to get the troops home in time for Thanksgiving — after the Pentagon also revealed it was shipping turkeys to the border to serve to forces who would not be allowed to go home.

Mattis told reporters earlier this month after he was questioned about the timing of the deployments: "We don’t do stunts in this department, thank you.” He visited the border briefly last week and used the opportunity to stress that he considered the deployment as a training opportunity, that the forces would not be in a law enforcement role, and that they would modify how they communicated the mission since they were still on U.S. soil.

Though Arizona Republican Sen. Jeff Flake, a past critic of the president, has called the border deployments a stunt, Sen. Joni Ernst, R-Iowa, and a retired National Guard member defended them as, “an opportunity for real, live training in their roles.

“Many of them, if they serve in logistics-type positions, they will actually be doing those missions on the border. So it is a very good skill,” Ernst, a SASC member, said Sunday on CBS’s "Face the Nation.” “As someone who has commanded troops, both in peace time and in wartime, to make sure that they keep their skills sharp. And so when they do deploy, they are in harm’s way overseas. They know exactly how to support the men and women on the front lines.”

On CNN last week, a combat-wounded veteran in Congress, Illinois Democratic Sen. Tammy Duckworth, criticized the border deployment as the latest sign of Trump’s lack of support for the military and “political statement.” The president was recently criticized for not holding a Veterans Day event on Monday and for skipping a visit to an American cemetery in France over the weekend to commemorate the end of the World War I.

“He did this as a political statement,” Duckworth said, “and it doesn’t help our troops, it doesn’t help readiness.”

With reporting by Tara Copp in Washington, D.C.