WASHINGTON — Sen. Tammy Duckworth doesn’t want National Guard troops deploying to the U.S. southern border for an unclear security mission. But if they are, she wants to make sure they’re getting paid properly for the work.

In a letter to Defense Secretary Jim Mattis and Homeland Security Secretary Kirstjen Nielsen on Wednesday, the Illinois Democrat expressed concerns that federal officials haven’t clarified the pay and benefits status of the roughly 4,000 guardsmen being deployed to southern states as part of President Donald Trump’s order earlier this month.

That mission, which has come under criticism from numerous Democrats including Duckworth, is designed to help border patrol agents better monitor border crossings. Guardsmen involved won’t perform law enforcement duties, but will assist in a array of security missions.

Duckworth said more specifics need to be made public to ensure that the troops involved are receiving the support they deserve.

play_circle_filled New details on Trump’s troop deployments to the Mexican border National Guard personnel will deploy on border security support missions, but will steer clear of law enforcement activities.

“Setting aside my serious reservations about the merits of this mission and the lack of evidence indicating a real operational need for this surge in personnel, my principle concern now is making sure that the Guardsmen are appropriately taken care of while activated and during post-mobilization,” the letter states.

“As you know, the length of activation and the manner they are called to duty will impact the benefits individuals are eligible for, (be they medical, educational, home loans or any other benefit military service entitles them to), and affect the stability of a service member’s home and work life.”

Arizona National Guard soldiers receive their reporting paperwork prior to deployment to the Mexico border at the Papago Park Military Reservation in Phoenix on April 9, 2018. Sen. Tammy Duckworth, D-Ill., asked federal officials on Wednesday to clarify the troops' status for pay and benefits issues. (Ross D. Franklin/AP)

× Fear of missing out? Sign up for the Early Bird Brief - a daily roundup of military and defense news stories from around the globe. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

In the letter, Duckworth requested the full cost and length of the mission, although White House officials have already declined to put an end date on the work. And she also expressed concern over Trump’s threat to withhold federal support from California National Guard officials over objections they raised to participating in the mission.

“By threatening not to fully fund the Guard, the government is effectively threatening not to look after the troops, their pay and benefits and their equipping needs,” the letter states. “Such a proposal to withhold federal dollars is unconscionable.”

Also on Wednesday, Maryland Democratic Rep. Anthony Brown, a member of the House Armed Services Committee, sent a similar letter to Mattis requesting “a comprehensive, long-term strategy for (troops’) role in southern border security.”

Both letters said the lack of mission clarity unnecessarily hurts service members caught in the political fight over whether to fund a wall along the southern American border.

At the time of Trump’s announcement, defense officials said guardsmen serving on the border would be activated under Title 32 status, which allows them to be paid for with federal dollars but remain under the authority of states’ governors.