Her department was responsible for implementing the proposed border wall and the separation of migrant families, and she became the face of the controversial moves.
She was also pushing back at an effort to house an "immigration" or "border czar" within Homeland Security, they said.
The policies Trump wants, and the outcomes he has promised, aren't within the power of the White House or the Department of Homeland Security. Both Nielsen and Alles were closely associated with former White House chief of staff John Kelly, who resigned previous year after falling out of favor with Trump.
The New York Times reported almost a year ago, in May 2018, that Nielsen was close to resigning after Trump berated her in front of other Cabinet secretaries over increases in the number of immigrants crossing illegally from Mexico. "They must have realized late last night how Trump incompetently botched the naming of her successor without having someone check the statute", Conway said.
Iowa Republican Sen. Chuck Grassley made both a public and private plea to the White House not to dismiss career homeland security officials.
But with Nielsen's departure, there will be no confirmed leader - and no one officially in charge of interior immigration enforcement, after the White House abruptly withdrew the nomination of Ron Vitiello to lead Immigration and Customs Enforcement last week.
In an administration already rife with record turnover, the rapid personnel changes could exacerbate internal conflict at the Homeland Security Department, the third-largest in the federal government, even as it grapples with what Nielsen and others have termed a "systemwide meltdown" in handling border security and immigration.
Meanwhile, Border Patrol agents are struggling to keep up with a historic flow of migrants.
Blamed migrant child's death while in DHS custody on her family.
O'Mara Vignarajah, head of LIRS, said that may reinforce Trump's interest in clamping down on asylum-seekers.
For now, she is working to wind down her role with the department before a Wednesday exit, and evaluating options for the future.
And so it seems as if Nielsen's un-resignation was for the sole goal of allowing the president time to remove Grady so that Grady would never be acting secretary, all to make good on a legally misbegotten tweet sent on a Sunday night.
Morgan said he doesn't believe the White House will resume family separations, but added that immediate action must be taken - even if it risks legal action.
Last Friday, the President visited Calexico, California, where he said, "We're full, our system's full, our country's full - can't come in!"
People familiar with immigration discussion said family separation is one of many suggestions that Trump and his aides were eyeing to tackle the problem of an ever-growing number of Central American families crossing into the U.S. The people were not authorized to speak publicly and spoke to The Associated Press on condition of anonymity.