What does that mean for 2020? At the moment, you could be excused for thinking that the only subject on Mr. Trump’s mind is the impeachment inquiry aimed at removing him from office.

But railing against immigration is in the president’s electoral DNA.

When Mr. Trump announced his 2016 campaign by calling Mexicans “rapists,” his political advisers were ecstatic, according to our reporting: “Trump had been at No. 8 in some polls when he was at the top of the escalator before his announcement. Within weeks, he had shot up to the top of the Republican pack.”

Stephen Miller, the architect of Mr. Trump’s immigration agenda, told us that voters would “resoundingly side with the political party that secured the border. And not by a little bit. Not by 55-45, 60-40, 70-30, 80-20. I’m talking 90-10 on that.”

Is that right? According to our reporting, one of the Republican Party’s leading pollsters concluded after the midterms that “immigration and the caravan had overwhelmed the economic message that Ryan and other Republicans had been trying to highlight by a two-to-one margin.”

That’s the danger for Mr. Trump. His obsession with immigration could once again overshadow the party’s tax cuts, a 3.5 percent unemployment rate and growth in wages.

Even so, expect dire warnings about “open border Democrats,” furious demands to stop asylum seekers and a relentless focus on his “big beautiful wall.” The president believes that immigration is what brought him to the White House, and he’s not about to abandon it now.