House Democrats are launching an investigation President Trump's "unprecedented" decision to remove much of the top leadership at the Department of Homeland Security.

Details: The letter — from House Judiciary Chairman Jerry Nadler, Homeland Security Chairman Bennie Thompson and Oversight Chairman Elijah Cummings — suggests White House senior aide Stephen Miller pushed for the resignation of Secretary Kirstjen Nielsen because she pointed out legal barriers to Trump's plan to reimplement child separation. Democrats are asking acting Homeland Security Secretary Kevin McAleenan to turn over any communications regarding the departure of top officials and all communications with Miller.

"As the Department's leaders have been fired or resigned, Mr. Miller appears to have taken on a greater role in leading the Administration's immigration policy. One report stated 'In a recent Oval Office meeting, Trump told Miller he would be in charge of handling all immigration and border affairs, according to officials familiar with the meeting."

The backdrop: A number of high-level officials at the Department of Homeland Security have exited this month, including Nielsen, acting Deputy Secretary Claire Grady and Secret Service chief Randolph "Tex" Alles. The shakeup comes as Trump plans to execute the most aggressive changes in immigration policy since his inauguration, in response to an ongoing surge of migrants at the border.