WASHINGTON — Stephen Miller was furious — again.

The architect of President Trump’s immigration agenda, Mr. Miller was presiding last month over a meeting in the White House Situation Room when he demanded to know why the administration officials gathered there were taking so long to carry out his plans.

A regulation to deny welfare benefits to legal immigrants — a change Mr. Miller repeatedly predicted would be “transformative” — was still plodding through the approval process after more than two years, he complained. So were the new rules that would overturn court-ordered protections for migrant children. They were still not finished, he added, berating Ronald D. Vitiello, the acting head of Immigration and Customs Enforcement.

“You ought to be working on this regulation all day every day,” he shouted, as recounted by two participants at the meeting. “It should be the first thought you have when you wake up. And it should be the last thought you have before you go to bed. And sometimes you shouldn’t go to bed.”

A few weeks after that meeting, the consequences of Mr. Miller’s frustration and the president he was channeling have played out in striking fashion.