Homeland Security Secretary Kirstjen Nielsen speaks with Border Patrol agents near a newly fortified border wall in California in October. (Gregory Bull/AP)

President Trump says he wants to get tougher on illegal immigration. Republican lawmakers are worried that he did the opposite with his Department of Homeland Security shake-up.

The Washington Post reports that Trump is furious about the record level of migrants coming to the border, a visual reminder that his campaign promise to severely slash immigration is unfulfilled.

On Sunday, Trump forced out Secretary Kirstjen Nielsen, the person who helped implement some of his most extreme border policies. Since then, he has continued to boot officials at the Department of Homeland Security — most recently the head of the Secret Service. There are reports that the head of U.S. Immigration and Customs Enforcement could be next. One official told The Post that this feels as if “they are decapitating the entire department.”

It’s a strange strategy for Trump, who seems to be casting out allies. As secretary, Nielsen vigorously defended her department, even as it separated families at the border and fired tear gas at crowds. If Trump doesn’t think any of that is tough enough, what is?

That’s the question that has Republicans in Congress worried. So worried that they’re going on the record with their concerns.

“They are the intellectual basis for what the president wants to accomplish in immigration,” Sen. Charles E. Grassley (Iowa), the most senior Senate Republican, said in an interview with The Post’s Seung Min Kim about the immigration officials Trump is ousting.

"Strikes me as just a frustration of not being able to solve a problem,” Sen. John Cornyn (R-Tex.) told Politico, adding, “I don’t know if there’s anybody who’s going to be able to do more.”

Before the firings, this was already a precarious issue for Republicans. Polling suggests that Republican base voters take immigration seriously: A December GW Politics poll found that immigration is the top issue Republican voters want Congress to deal with this year. But Congress hasn’t tackled any major immigration issues yet, and Trump may have just made it harder for lawmakers to do so by throwing a key agency into chaos.

Also not helpful from Republicans’ perspective is that Trump seems committed to veer as far to the right on immigration as politically possible. That may have helped him win the presidency, but it’s not where the battle for control of Congress in 2020 is going to play out. It will be fought in swing districts such as Republican Rep. Will Hurd’s in Texas along the border, where the electorate is much wider than the small slice of immigration hard-liners Trump is catering to. (Surprise: Hurd does not seem to be a fan of Trump’s DHS shake-up.)

How hard-line to be on immigration isn’t a new tension between Republicans in Congress and Trump. A number of them opposed his national emergency declaration at the border, a move he made only after Congress refused to fund construction of the wall.

But the party’s relationship over immigration has perhaps never been as fraught as it is now. Kim reports that Grassley had plans to go on Fox News Channel and broadcast directly to the president why he’s making a mistake by gutting his main immigration-enforcement agency.

Republicans in Congress have worked with Trump enough to understand that the president mostly looks out for his own political career, even at the expense of the rest of his party. But even when viewed through the lens of Trump’s own political health, a number of Republicans think he just made a big mistake.