Democrats saw former Homeland Security Secretary Kirstjen Nielsen as the face of cruelty when it comes to the Donald Trump administration's immigration policy, the woman who presided over the separation of families seeking refuge in the United States.

Nielsen is gone, along with several other top DHS officials who abruptly announced their resignations – or had their departures announced for them – last week. But House majority Democrats are turning their critical eyes at Stephen Miller, the 33-year-old White House adviser who is now driving what critics see as an increasingly hard-line immigration policy.

Miller was reportedly the author of the idea of sending foreign detainees to so-called "sanctuary cities" to needle Democratic-governed jurisdictions which have declared they will limit their cooperation with federal authorities seeking to round up and deport immigrants in the country illegally. According to a report in The New York Times, Miller became audibly agitated and frustrated in a conference call with administration officials, angry that so many people seeking refuge in the United States were passing the first legal hurdle of having a "credible fear" of persecution if they were sent back to their home countries. Miller, the Times said, citing others on the call, seemed to want immigration authorities to find a way around the law Congress wrote.

Miller is also known for his startling remarks in the early months of Trump's presidency that "the powers of the president to protect our country are very substantial and will not be questioned."

Democrats, who have been using their new House majority status to conduct oversight and investigations of a slew of Trump-related matters, want to bring Miller before them for questioning on immigration.

"Steve Miller, who seems to be the boss of everybody on immigration, ought to come before Congress and explain some of these policies," House Judiciary Committee Chairman Jerrold Nadler, Democrat of New York, said on CNN's "State of the Union" on Sunday.

Nadler acknowledged that Miller might claim executive privilege to avoid testifying before the panel, but "that would be a misuse of executive privilege," the chairman said. "Because he seems to be making the decisions, not the Cabinet secretaries who come and go."

Immigration Cartoons View All 171 Images

That last remark appeared aimed at Trump, who does not like to be upstaged by people who work for him. Notably, while the White House at first dismissed the idea of sending detainees to sanctuary cities as something barely discussed, Trump later on Friday claimed ownership of the plan. On Saturday, the president tweeted a "demand" that places like California accept such detainees.

"Just out: The USA has the absolute legal right to have apprehended illegal immigrants transferred to Sanctuary Cities. We hereby demand that they be taken care of at the highest level, especially by the State of California, which is well known or its poor management & high taxes!" Trump said on Twitter.

Democrats, Trump added, must change immigration laws "fast. If not, Sanctuary Cities must immediately ACT to take care of the Illegal Immigrants – and this includes Gang Members, Drug Dealers, Human Traffickers, and Criminals of all shapes, sizes and kinds. CHANGE THE LAWS NOW!"

House Speaker Nancy Pelosi, Democrat of California, dismissed the idea of sending detainees to sanctuary cities as "unworthy of the presidency." Nadler, on CNN, also criticized the suggestion.

"He shouldn't use them as what he imagines as retribution to political opponents in various areas. It's another misuse of presidential power, against the law," Nadler said.

Trump has been frustrated by both Congress and the courts on immigration during his presidency. When Republicans ran both chambers of Congress, lawmakers did not change immigration laws to Trump's dictates. Further, courts have thwarted many of his policies: Most recently, last week, a federal court said the administration could not force would-be refugees to apply for asylum in Mexico but must allow them to ask for refugee status once they crossed the border into the United States. That ruling has been stayed while it is under appeal.

Part of Trump's frustration is rooted in different re-election strategies. The president made a border wall and restrictions on both legal and illegal immigration central to his 2016 campaign, and he must explain to his base next year why he has been unable to accomplish those goals.