A top Republican senator has warned Donald Trump to halt an apparent purge of the top ranks of the Department of Homeland Security, amid forced resignations, a renewed hardening of the White House stance over immigration and accusations that the president is manufacturing a declared crisis at the US-Mexico border.

The Iowa senator Chuck Grassley, the longest-serving Republican in the US Senate, told the Washington Post he was urging Trump to save the job of Lee Francis Cissna, the director of US Citizenship and Immigration Services, who is reportedly in the president’s crosshairs just two days after the forced resignation of Kirstjen Nielsen as homeland security secretary.

“The president has to have some stability, and particularly with the number one issue that he’s made for his campaign, throughout his two and a half years of presidency,” Grassley said. “He’s pulling the rug out from the very people that are trying to help him accomplish his goal.”

Sign up for the US morning briefing

Trump, reportedly acting on the advice of his hardline immigration adviser, Stephen Miller, ousted Nielsen at the weekend, and on Monday announced the departure of the director of the Secret Service, Randolph Alles.

Last week, Trump abruptly withdrew his nominee to lead Immigrations and Customs Enforcement, saying he wanted someone “tougher”.

Next out the door could be Cissna and the DHS general counsel, John Mitnick, several news outlets have reported.

Trump is “becoming increasingly unhinged about the border crisis”, a senior administration official told CNN, with the president demanding actions such as blocking all migrants from seeking asylum, which top DHS officials resisted because it would be illegal, and revisiting last spring’s policy of family separations, which caused chaos and widespread uproar.

Grassley told the Post he was “very, very concerned” about reports that Cissna could be ousted next. He said he texted the acting White House chief of staff, Mick Mulvaney, to express his concerns, and was appearing on Fox News in an effort to get the attention of the president, who is an avid viewer of the network.

“One, those are good public servants,” Grassley said. “Secondly, besides the personal connection I have with them and the qualifications they have, they are the intellectual basis for what the president wants to accomplish in immigration.”

Grassley also criticized the growing influence of Miller within the administration. “I think it would be hard for him to demonstrate he’s accomplished anything for the president,” he told the Post.

The Senate homeland security committee is set to hold a hearing on Tuesday morning on the immigration situation at the southern border.