According to Trump, the head of U.S. Customs and Border Protection (CBP), Kevin McAleenan — the country's law enforcement agency at the border and at ports of entry — will temporarily take charge of DHS as acting secretary, which would mean a change in leadership at CBP as well.
Heading in a 'tougher direction'
The top-down shake-up is said to be motivated by Trump's interest in more restrictions regarding migration at the U.S.-Mexico border, and with immigration overall.
In rescinding Vitiello's appointment last week, Trump said, "We want to go in a tougher direction" on immigration but did not elaborate.
Nielsen's departure comes after publicly conflicting with the president late last month over U.S. relations with Central America, and amid media reports that Nielsen did not go far enough in pushing Trump's restrictionist agenda at the southern U.S. border.
"Secretary Nielsen's had a rocky tenure… from denying family separations were initially happening to having to justify the 'zero-tolerance' policy," said Krish O'Mara Vignarajah, president of Lutheran Immigration and Refugee Services. "This wasn't altogether unexpected."
With media reports that Trump wants to reinstate a policy of separating migrant children from their families at the border, the White House on Monday did not issue a flat-out denial of the allegation.
Hogan Gidley, White House deputy press secretary, told reporters: "The separation of families, you know, the president has said before he does not like that. It's a horrible practice. But Congress has a way to fix that so that it will not be a magnet for people to come here and use children to do it."
But migration is not triggered by one variable, such as congressional action, rather by several: conditions in migrants' home countries, policies in the United States, economic variables, weather. And that list changes.
Neither Nielsen nor Trump, however, have publicly acknowledged that the administration's policies may in fact be contributing to the increased number of border-crossers in recent months, as Dree K. Collopy, chair of the National Asylum and Refugee Liaison Committee of the American Immigration Lawyers Association, suggested in January.
Democrats welcome Nielsen resignation
News accounts say Nielsen had no intention of quitting when she arrived at the White House on Sunday to meet with Trump, but that he was determined to ask for her resignation, which she submitted shortly after the meeting.
White House sources have said Trump often yelled at Nielsen for apparently not being strong enough in curbing the number of migrants trying to enter the United States.
"It is deeply alarming that the Trump administration official who put children in cages is reportedly resigning because she is not extreme enough for the White House's liking," House Speaker Nancy Pelosi said in a statement following Nielsen's announcement.
Chairman of the Congressional Hispanic Caucus, U.S. Rep. Joaquin Castro, responded by summarizing Nielsen's tenure at DHS as "championing President Trump's cruel anti-immigrant agenda" and McAleenan's appointment "deeply disturbing" given the CBP commissioner's actions at the border.
Castro went on to say McAleenan "cannot be trusted… based on his record of prioritizing Trump's harmful policies."
But Nielsen's removal and McAleenan's temporary appointment are not a slam dunk on either side of the political spectrum. Noted immigration restrictionist Mark Krikorian, head of the Center for Immigration Studies, tweeted that he is "not sure McAleenan would be an improvement over Nielsen."
Trump has expressed frustration with the situation along the southern border, where hundreds of thousands of migrants trying to escape poverty and crime in Guatemala, El Salvador and Honduras have traveled through Mexico in hopes of entering the United States. Under U.S. law, foreign nationals are allowed to apply for asylum.
Nielsen's last day in office will be Wednesday, April 10.