After President Donald Trump announced on Friday night the resignation of Department of Homeland Security Acting Secretary Kevin McAleenan, immigrant rights advocates vowed that "we will not forget his cruelty."

Families Belong Together, a national coalition of nearly 250 organizations, tweeted that McAleenan "was never fit to lead" the department and, as its chief, "has caused tremendous suffering in our communities."

BREAKING: Kevin McAleenan was never fit to lead @DHSgov. Under his watch, thousands of children were separated from their families. McAleenan has caused tremendous suffering in our communities. We will not forget his cruelty. https://t.co/t2ZaO3bErZ — Families Belong Together (@fams2gether) October 12, 2019

Trump, who disclosed McAleenan's departure in a pair of tweets on Friday, said he would announce a replacement next week. "Kevin McAleenan has done an outstanding job" and "we have worked well together," Trump wrote. "Kevin now, after many years in Government, wants to spend more time with his family and go to the private sector."

According to New York Times White House correspondent Maggie Haberman, who tweeted about the development on Friday, "Despite the president tweeting the news, McAleenan was not forced from his job. He made the decision, went to the WH today with letter, per people familiar with the events, and told the president tonight by phone."

McAleenan, in his own statement shared on Twitter Friday night, thanked Trump "for the opportunity to serve alongside the men and women of the Department of Homeland Security" and said that he "will work with the White House and DHS leadership teams on a smooth transition."

Rights advocates and Democrats in Congress responded by highlighting McAleenan's role in imposing the Trump's widely condemned immigration policies and pointing out that this is just the latest in a long series of departures from administration.

"Since Day One, this administration has inflicted an anti-family and anti-immigrant agenda marked by extraordinary cruelty and chaos on our nation," Democratic House Speaker Nancy Pelosi (Calif.) said in a statement Friday.

"The departure of the acting Homeland Security secretary is the latest sign of this administration's failed leadership, which has worsened the humanitarian situation at the border, injected pain and tragedy into countless lives, and done nothing to improve the situation at the border," she added. "The administration's barbaric family separation policy and the appalling conditions facing children and families at the border will leave a dark and enduring stain on our nation for years to come."

Under McAleenan, cruel & dangerous policies targeting children & families fleeing persecution have expanded further than ever. I wish we could view this moment as the turning of a new leaf. But I have little hope that will come from anyone who would accept Trump’s nomination. — Senator Jeff Merkley (@SenJeffMerkley) October 12, 2019 So goes another DHS head who bore responsibility for Trump’s immoral immigration policies, including family separation and MPP. With no Senate-confirmed leadership left at DHS, the Administration is demonstrating once again its disdain for the rule of law. https://t.co/1fiIf6ieYy — (((Rep. Nadler))) (@RepJerryNadler) October 12, 2019

McAleenan, a career civil servant who worked under both Democratic and Republican administrations, took the helm of DHS in April after Secretary Kirstjen Nielsen left.

As The Associated Press outlined Friday, "despite his stance as a moderate Democrat who pushed for aid to be restored to Central American nations," McAleenan was also a key player in many Trump policies that have elicited substantial criticism.

He was among those behind the administration's widely maligned practice of separating families at the border last year, though McAleenan later said he regretted the policy because it lost the public trust. He also expanded a program where asylum seekers are forced to wait their claims out in Mexico; more than 42,000 migrants have been subjected to it. And most recently, the administration made migrants ineligible for asylum if they crossed through a third country on their way to the U.S. And he brokered major agreements with Central American countries on asylum and border security—something others were unable to do.

Detailing McAleenan's months-long tenure as head of DHS, Haberman and two Times colleagues reported:

While Mr. McAleenan had the strong backing of Jared Kushner, the president's son-in-law and senior adviser, he clashed over personnel decisions with Stephen Miller, a White House aid and the architect of Mr. Trump's immigration agenda. In recent months, Mr. McAleenan grew increasingly irritated by the harsh language used by agency officials installed by the White House. Kenneth T. Cuccinelli II, who once advocated an end to birthright citizenship, was installed to lead the agency that manages legal immigration. Mark Morgan, who once said he could determine future gang members by looking at detained migrant children, was selected to oversee Immigration and Customs Enforcement. He then replaced John Sanders, an ally of Mr. McAleenan, to lead Customs and Border Protection. Both are serving in acting positions.

Given those tensions, Cristina Jiménez, co-founder and executive director of the youth advocacy group United We Dream, expressed concern about what immigration policies will look like in the wake of McAleenan's departure. "Let's be clear. This is NOT a victory," she tweeted. "This confirms that the person leading this administration's nativist immigration policy/agenda is white supremacist Stephen Miller."

“We’ll never know all the outlandish policies McAleenan probably had to avert, redirect, or quietly diffuse during his tenure at DHS. Now, without him there, we’re about to find out.” https://t.co/W5TsQaOxeR — Jonathan Blitzer (@JonathanBlitzer) October 11, 2019

Although McAleenan sometimes clashed with Trump allies on immigration policy, rights advocates still held him accountable for his role in the administration. As Common Dreams previously reported, "McAleenan was shouted offstage Monday at Georgetown University by pro-immigrant demonstrators who said 'Trump henchmen' shouldn't be 'given a platform to spread hatred or defend the racist, xenophobic policies' of the administration."