Updated 6:59 p.m. | President Donald Trump announced Sunday that Homeland Security Secretary Kirstjen Nielsen is leaving his administration.

The move by Trump comes after months of frustration with what he saw as her inability to clamp down on illegal crossings at the U.S.-Mexico border.

After tweeting  that Nielsen was leaving, Trump also revealed that Kevin McAleenan, the current U.S. Customs and Border Protection Commissioner, will become the acting Homeland Security secretary.

“I have confidence that Kevin will do a great job!” the president said on Twitter.

….I am pleased to announce that Kevin McAleenan, the current U.S. Customs and Border Protection Commissioner, will become Acting Secretary for @DHSgov. I have confidence that Kevin will do a great job!

— Donald J. Trump (@realDonaldTrump) April 7, 2019

Nielsen participated in two events with Trump on Friday at the U.S.-Mexico border in Southern California. And during her remarks at a roundtable in Calexico, she was open about the uptick in attempted illegal border crossings and apprehensions that has frustrated the president in recent weeks just when he wants to make immigration a major 2020 re-election theme.

“Groups, as you know, are going up.  It used to be that we’d see, maybe, one group a year of over 100.  We’ve already seen over 100 groups of over 100 people, which is 100 reaching our border at one time that the Border Patrol goes and picks up,” she said.

Nielsen’s departure means the president is adding another potentially contentious nomination to the Senate’s docket. But he also has begun to simply name acting secretaries and senior aides, saying that doing so gives him an undefined kind of leeway that he doesn’t get with Senate-confirmed senior officials.

“I have ‘acting.’ And my ‘actings’ are doing really great,” the president said Jan. 6 as he left the White House for a border security summit with senior aides at Camp David. “But I sort of like ‘acting.’”

“It gives me more flexibility. Do you understand that?” he told reporters. “So we have a few that are acting. … If you look at my Cabinet, we have a fantastic Cabinet. Really good.”

Steve Vladeck of the University of Texas School of Law told Roll Call in January that the number of “actings” is legally “murky, in the sense we’ve never seen it on this scale.”

“To me, the controversy arises from the confluence of several things: One, how many there are? Two, how long each one has been serving and probably will serve? Three, which positions we’re talking about? For example, it’s virtually unheard of to have an acting secretary of Defense. And four, all of this is happening while the president has a deeply supportive Republican majority in Senate.”

Last month, Nielsen was touting that the administration was on track for more than 900,000 apprehensions at the southern border.

During her tenure, she was asked again and again to implement and publicly defend Trump’s most brazen immigration policies. It often brought loud objections from congressional Democrats.

For instance, last June, Nielsen caught the ire of Democrats after she tweeted that the Trump administration did “not have a policy of separating families at the border.”

New York Rep. Hakeem Jeffries called Nielsen a “stone cold liar.” Reps. Sheila Jackson Lee of Texas and Katherine M. Clark of Massachusetts also called her statement a “lie.” California Rep. Eric Swalwell said, “She knows we can check this, right?”

One of the defining moments of her tenure came on June 18 last year, when the White House was under fire over its policy of separating families and detaining young migrant children in chain-link rooms critics called “cages.”

Nielsen was on the road, giving a long-planned speech in New Orleans; White House Press Secretary Sarah Huckabee Sanders wanted her to field reporters’ questions about the policy.

The briefing was delayed three times as Nielsen flew back to Washington, and then huddled with White House officials. When she finally appeared in the briefing room, Nielsen delivered an uneven performance, frequently contradicting herself and using dubious statistics.

Nielsen quickly faced tough questions, including whether the current practice was not child abuse or akin to the internment of Japanese Americans during World War II. At one point, a reporter played a ProPublica video on his mobile device depicting the cries and screams of children who were being separated from parents or adults who brought them to the border. The DHS chief appeared shaken, and struggled at times to provide clear answers.

Nielsen and other senior administration officials used a 2008 child-trafficking law and a subsequent court ruling to justify the policy, saying both left the administration with its hands “tied” and compelled law enforcement officials to send migrant children to one processing center and adults to another for the misdemeanor offense of crossing the border.

[Trump Predicts ‘Deal-Making,’ Many Fights Ahead With Democrats]

At other times, the former DHS secretary appeared unwilling to defend her boss by sidestepping questions from lawmakers about his reported conduct.

For instance, in January 2018, Nielsen told the Senate Judiciary Committee she “did not hear” if Trump referred to Haiti and African nations as “shithole countries” in a meeting with House and Senate lawmakers.

“The president used tough language in general, as did other congressmen in the room,” she told the senators.

“What I heard him saying is that he would like to move away from a country-based quota system to a merit-based system,” she said. “I don’t specifically remember a characterization of countries in Africa.”

But her frequent defenses of Trump and promotion of his hard-line immigration policies rankled Democratic lawmakers and administration critics. Last July, she was shouted out of a Mexican restaurant in Washington.

Perhaps in a bit of foreshadowing, Trump on Friday had very little to say to or about Nielsen at both border events. And he never applauded her performance during what was a rocky tenure.

“I’d like to thank Secretary Nielsen for being here,” he said at the start of the roundtable in California.

Watch: How and Why Many Key Officials Have Exited the Trump Administration