Sen. Tammy Duckworth Ladda (Tammy) Tammy DuckworthMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled Biden courts veterans amid fallout from Trump military controversies John Fogerty: 'Confounding' that Trump campaign played 'Fortunate Son' at rally MORE (D-Ill.), an Army veteran, called on 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 to testify before Congress regarding the contents of his resignation letter in which he acknowledged policy differences with the president.

Secretary Mattis should testify before Congress and answer questions about the alarming contents of his resignation letter. — Tammy Duckworth (@SenDuckworth) December 21, 2018

“Secretary Mattis should testify before Congress and answer questions about the alarming contents of his resignation letter,” Duckworth said on Twitter. “Congress should use its power to check this President, defend our alliances and hold our adversaries accountable when Donald Trump refuses to.”

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It is unclear in what format such a testimony would take place.

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 said Thursday that Mattis will leave the administration in February. The announcement was met with bipartisan concern on Capitol Hill, as many had seen the former Marine Corps general as a stabilizing force in the White House.

Mattis issued a stunning resignation letter devoid of any praise for Trump and acknowledging multiple policy disagreements they had.

“Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” Mattis wrote in the letter.

Trump and Mattis were known to butt heads on a multitude of issues, including Trump’s stances toward NATO and Russia, banning transgender troops from serving in the military, canceling joint U.S.-South Korea military exercises, withdrawing from the Iran nuclear deal and deploying troops to the U.S.-Mexico border.

His departure has long been expected after accounts of disagreements with Trump became more widespread and the president suggested a staff shakeup was imminent following November’s midterm elections.

However, his blatant disagreements with the president startled many on Capitol Hill.

“I was sorry to learn that Secretary Mattis, who shares those clear principles, will soon depart the administration. But I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America’s global leadership,” Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellFEC flags McConnell campaign over suspected accounting errors Poll: 59 percent think president elected in November should name next Supreme Court justice Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ky.) said in a statement.