WASHINGTON (Reuters) - Stephen Miller, the White House aide driving a hardline immigration stance, was invited on Wednesday to testify to a House committee about the Trump administration’s policy of separating migrant children from their parents at the border.

FILE PHOTO: White House adviser Stephen Miller walks across the tarmac to board Air Force One as he departs Washington with U.S. President Donald Trump for travel to Grand Rapids, Michigan from Joint Base Andrews, Maryland, U.S., March 28, 2019. REUTERS/Joshua Roberts

Representative Elijah Cummings, chairman of the Democratic-led House of Representatives’ Oversight Committee, wrote in a letter that he would like Miller to explain why the administration decided to separate immigrant children from their parents at the border.

He also called for explanation of “transferring asylum seekers to sanctuary cities as a form of illegal retribution against your political adversaries, and firing top administration officials who refuse orders to violate the law.”

The White House did not immediately respond to a request for comment.

Miller has been a driving force behind President Donald Trump’s immigration policies. A former Senate aide, Miller joined Trump’s campaign early in the primary season and has helped shaped much of the administration’s domestic policy.

The committee’s request stops short of demanding Miller appear. The panel could exercise its power to subpoena him, although Miller could likely invoke executive privilege because his discussions with Trump may not be subject to compulsory testimony.

Trump said earlier this month that he would not revive a “zero tolerance” prosecution policy his administration previously implemented that required adults crossing the border illegally to be criminally prosecuted.

Since children could not be held in federal jails with their parents, the policy led to the separation of parents from thousands of their young children with whom they traveled.

The border enforcement stance prompted legal challenges and a public outcry that eventually forced the policy’s reversal.

Under Trump, federal agencies are trying to stem rising numbers of people arriving at the border, many of them families fleeing violence and poverty in Central America, that swelled last month to the highest in a decade.

Trump campaigned in 2016 on a promise to restrict immigration and has made it a central focus of his presidency.