WASHINGTON - The chairman of the House Oversight Committee on Wednesday asked White House senior adviser Stephen Miller to testify before the committee next month about Trump administration policies on immigration.

In a letter, Rep. Elijah Cummings, D-Md., requested that Miller appear before the panel on May 1 to answer questions about what Cummings called "troubling" immigration decisions, such as the past policy of separating migrant children from their parents and the recent personnel upheaval at the Department of Homeland Security.

Homeland Security officials told The Washington Post that Miller was behind the White House plan to bus undocumented migrants to the districts of political adversaries and other Democrats who represented “sanctuary cities.” Miller has also been reported to have great influence over the White House’s hard-line immigration policy positions.

Democrats have increasingly focused on Miller. The White House aide is not in a Cabinet-level post, yet Democrats worry that he is calling the shots more than other department heads who work on immigration on a daily basis.

“I am inviting you to testify before the committee because it appears that you are one of the primary moving forces behind some of the most significant - and in my view, troubling - immigration policies coming out of the Trump White House,” Cummings wrote.

Cummings said in the letter that he would ask Miller about why the Trump administration sought to separate immigrant children from their families as well as President Donald Trump's move to force out top immigration officials at DHS and agencies within.

It is unlikely the White House would allow a top aide to testify, and in the letter, Cummings acknowledged that White House advisers do not testify before congressional committees. Cummings sought to argue against that rationale in his letter.

“I understand that you may not want to submit yourself to rigorous questioning by a full panel of committee members, some of whom have grave concerns about your views and your actions,” Cummings wrote. “However, since President Trump recently announced that you are ‘in charge of handling all immigration and border affairs’ it makes sense for Congress to hear directly from you about how federal agencies are implementing your policies.”