After sending more than 13,000 Twitter messages in less than three years, Jon Feere, an outspoken opponent of illegal immigration, suddenly went silent after Inauguration Day.

As a legal policy analyst at the Center for Immigration Studies, a Washington-based group that favors significant reductions in immigration, Mr. Feere had staked out tough positions on the subject, including pushing for an end to automatic citizenship for children born in the United States.

Mr. Feere’s newfound reticence reflected not a change of heart but a new employer. He now works for Immigration and Customs Enforcement, the agency tasked with finding and deporting people living in the United States illegally.

His last Twitter post, on Jan. 20, read simply: “It’s time to make immigration policy great again.”

For years, a network of immigration hard-liners in Washington was known chiefly for fending off proposals to legalize the status of more people. But with the election of a like-minded president, these groups have moved unexpectedly to offense from defense, with some of their leaders now in positions to carry out their agenda on a national scale.

“We’ve worked closely with lots of people, who are now very well placed in his administration, for a long time,” said Dan Stein, president of the Federation for American Immigration Reform, another hard-line group.

Julie Kirchner, who served for a decade as executive director of the organization, also known as FAIR, is now working as an adviser to the commissioner of Customs and Border Protection. Kellyanne Conway, before she was known for campaign work and spirited defenses of Mr. Trump on cable television, worked regularly as a pollster for FAIR.

Mr. Trump’s senior White House adviser, Stephen Miller, worked tirelessly to defeat immigration reform as a staff member for Senator Jeff Sessions, now the attorney general. Gene P. Hamilton, who worked on illegal immigration as Mr. Sessions’s counsel on the Judiciary Committee, is now a senior counselor at the Department of Homeland Security, the parent agency of the Border Patrol and ICE, where Mr. Feere is working. Julia Hahn, who wrote about immigration for Breitbart — with headlines like “Republican-Led Congress Oversees Large-Scale Importation of Somali Migrants” — has followed her former boss, Stephen K. Bannon, to the White House as a deputy policy strategist.

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.

“Trump has put together the people who are taking this thing down to the operating-instruction level,” Mr. Stein said.

Even those who have labored for decades to scale back immigration did not expect such a dramatic change. “This is inconceivable a year ago,” said Mark Krikorian, executive director of the Center for Immigration Studies. “Frankly, it’s almost inconceivable six months ago.”

When Mr. Feere asked him for a leave of absence to work on the Trump campaign, Mr. Krikorian said he granted it without necessarily expecting it to lead anywhere. “Honestly, I didn’t think that would pan out,” he said, but recalled telling Mr. Feere, “Look, you know we’ve always got a job for you if it doesn’t work out.”

The groups’ growing influence has also brought renewed scrutiny to their inflammatory statements and shared nativist roots. The Center for Immigration Studies, FAIR and another group, the grass-roots organizer NumbersUSA, all were founded or fostered in their early stages by the activist John Tanton, a Michigan ophthalmologist who had an outsize influence on the immigration debate through his organizing efforts.

Dr. Tanton came under sharp criticism for corresponding with white nationalists and for couching the fight to reduce immigration as a racial and demographic struggle. “For European-American society and culture to persist requires a European-American majority, and a clear one at that,” Dr. Tanton once wrote to a friend, elsewhere expressing his fear of a “Latin onslaught.”

The Southern Poverty Law Center has been quick to point out how the Center for Immigration Studies has circulated articles “penned by white nationalists, Holocaust deniers, and material from explicitly racist websites,” and added the immigration center to its list of active hate groups. Mr. Krikorian has spoken out against the label, saying it served only to shut down legitimate debate on immigration.

The Southern Poverty Law Center has long been especially critical of FAIR, which had in the past received money from the Pioneer Fund, a foundation that has financed research on the relationship between race and intelligence. Mr. Stein of FAIR rejected the attacks as politically motivated, prompted by the group’s success in helping defeat an immigration overhaul in Congress.

This month, FAIR filed a complaint with the Internal Revenue Service accusing the Southern Poverty Law Center of committing “flagrant and intentional” violations of its tax-exempt status by criticizing Republican candidates during the 2016 presidential race.

Richard Cohen, president of the Southern Poverty Law Center, said it never crossed the line into improper political activity. “I think we have an obligation to expose hate not just in the dark corners of our society but also in the mainstream,” Mr. Cohen said. “We’ve gotten under FAIR’s skin many times, and now they feel like they have allies in the administration and they’re going for it.”

Although immigration advocates call them xenophobic, people at all three groups say they do not like to be labeled anti-immigrant; the Center for Immigration Studies uses the motto “low immigration, pro-immigrant” on its website. They say they just expect to see the nation’s immigration laws enforced and that those living here illegally are caught and deported.

They also say they want legal immigration brought down to what they view as more sustainable levels, in particular to help buoy the wages of lower-income Americans who compete with unskilled migrants on the bottom rungs of the work force.

“The average American basically likes the idea of immigration, maybe loves the concept — it’s played an important historic role in our history — but would be perfectly fine if we didn’t have another immigrant for 50 years,” Mr. Stein said.

FAIR lobbies members of Congress and their staff from its offices on Massachusetts Avenue, a short walk from Capitol Hill, while maintaining strong contacts with talk-radio hosts. There is even a radio studio in the group’s office.

With roughly two dozen staff members and fellows, the Center for Immigration Studies provides research, filling the traditional think-tank role.

NumbersUSA is perhaps best known for exhorting members to overwhelm senators with faxes — more than a million were sent — during an effort in 2007 to pass a bill offering a path to citizenship for millions of illegal immigrants and creating a new temporary worker program. The group likes to point out that it has “activists in every congressional district,” as the group’s founder, Roy Beck, put it in a recent interview at its office in Arlington, Va. NumbersUSA now claims eight million “participants” between its Facebook followers and email lists.

All three receive small donations from individuals but also millions of dollars in recent years from the Colcom Foundation, a Pittsburgh-based organization founded by Cordelia Scaife May, a Mellon banking heiress, which has given heavily to anti-immigration causes. Her brother, Richard Mellon Scaife, was well known for bankrolling conservative causes and attacks on Bill and Hillary Clinton.

Despite their recent policy victories, the groups remain wary as to whether the administration will follow through on all its promises. In particular they point to Mr. Trump’s failure to immediately end President Barack Obama’s policy of Deferred Action for Childhood Arrivals, allowing the so-called Dreamers who came illegally as children to remain in the country, as well as his choice of a pro-immigation economist to lead his Council of Economic Advisers.

“We feel like we are going to continue to need to bring grass-roots influence on this administration because there’s a lot of competing interests,” said Mr. Beck of NumbersUSA.

At the same time, he pointed to a list of 10 priorities that NumbersUSA put out last summer for strengthening enforcement, and noted that the Trump administration had already addressed eight of them. One of the other two is ending birthright citizenship for children whose parents are not citizens, a controversial idea that would most likely require a constitutional amendment.

“The biggest enemy we face right now is complacency,” Mr. Stein of FAIR said, “because Trump’s people have our ideas.”