Daniel Tichenor, an immigration politics scholar at the University of Oregon, called it “highly unusual” in the post-World War II era to have proponents of sharply reduced immigration in such high-ranking positions.

“You would have to go to the 1920s and 1930s to find a comparable period in which you could point to people within the executive agencies and the White House who favored significant restrictions,” Mr. Tichenor said.

Their influence is already being felt. Mr. Trump is known for his sound-bite-ready pledges to deport millions of people here illegally and to build a border wall, but some of the administration’s more technical yet critical changes to immigration procedures came directly from officials with long ties to the hard-line groups.

These include expanding cooperation between immigration agents and local law enforcement officials; cracking down on “sanctuary cities”; making it more difficult for migrants to successfully claim asylum; allowing the Border Patrol access to all federal lands; and curtailing the practice of “catch and release,” in which undocumented immigrants are released from detention while their cases plod through the courts.

Although his proposed budget slashed $1 billion from the Justice Department, Mr. Trump included $80 million to hire new judges to accelerate deportation proceedings. Mr. Sessions said at an event at the border in Arizona this month that 50 would be added to the bench this year and 75 more next year.