President Trump speaks with reporters about border policy during a fundraising event in San Antonio on Wednesday. (Evan Vucci/AP)

Senate Republicans are pressuring President Trump not to fire any more Homeland Security officials, warning that the department can’t function properly if the White House continues to oust key members of its leadership team.

Homeland Security Secretary Kirstjen Nielsen is leaving the agency on Wednesday after submitting her resignation following a private meeting with Trump on Sunday. She is being replaced on an acting basis by Kevin McAleenan, the former commissioner of U.S. Customs and Border Protection.

The White House announced Monday that Randolph “Tex” Alles, the director of Secret Service, would be leaving his position “shortly.” And both U.S. Citizenship and Immigration Services Director Lee Francis Cissna and DHS’s general counsel, John Mitnick, could be the next to go, according to department officials.

The moves at DHS come as Trump is furious about the department’s inability to reduce illegal immigration to the United States, one of his signature campaign promises. A surge of migrant families from Central America in recent weeks is taxing the government’s ability to process asylum seekers.

The migrants say they are fleeing violence and poverty in their countries, but Trump described the situation at the border in dark terms during a roundtable Wednesday with supporters in San Antonio.

“Dangerous people are coming here and the good people are dying,” he said. “This has nothing to do with politics, this has nothing to do with campaigning . . . Who the hell can live like this?”

Republican senators remain concerned that Trump’s anger will lead to a continuing purge of DHS officials with no clear replacements.

Sen. Ron Johnson (R-Wis.), chairman of the Homeland Security and Governmental Affairs Committee, said he is worried that Mitnick could be dismissed. Johnson is preparing a letter to the administration advising “against even considering getting rid of the counsel.”

Johnson emphasized that Mitnick was a consensus pick, having been confirmed unanimously to the job.

“You need management in the department and particularly a counsel that will shoot straight,” Johnson said. “I just think the current counsel is a good person and we want to make sure he stays on the job.”

He said McAleenan is facing a job that is an “almost impossible task for anybody.”

“And it’s not getting any easier whatsoever,” Johnson said. “But right now, I’ve got a great deal of confidence in the commissioner.”

Another GOP senator, Sen. Charles E. Grassley (Iowa), has contacted top administration officials, including senior White House adviser Jared Kushner, acting White House chief of staff Mick Mulvaney and McAleenan, to push them to keep Cissna in his job.

Grassley worked closely with Cissna when he was detailed from the administration to the Senate Judiciary Committee when the Iowa Republican served as its chairman.

Despite his lobbying, Grassley said he had not yet received direct assurances that Cissna would stay in his post.

“I don’t know whether I’ll be satisfied or not when it’s all done,” Grassley said. “I’m beyond the point of considering whether or not I’m going to be satisfied.”

Later Wednesday, acting White House chief of staff Mick Mulvaney and McAleenan went to Capitol Hill to meet privately with a half-dozen Senate Democrats in a meeting the administration requested to discuss the migration influx.

“The situation at the border is challenging and we’re talking about what can be done,” said Sen. Richard J. Durbin (D-Ill.). Mulvaney declined to comment as he left the meeting.

Also on Wednesday, Nielsen announced that Ronald Vitiello was stepping down as acting Immigration and Customs Enforcement director. Last week the White House pulled his nomination to lead the agency full time — a move that caught senators, including Johnson, by surprise. Trump said he decided he wanted someone who would take a “tougher” approach for the job.