Although conservatives in Congress are vowing to attack the president’s executive action on immigration by blocking the funding for it, plans for the small army of workers are moving forward. The action is part of a larger trend: From 2001 to 2012 — mostly after the Sept. 11 attacks — the government added about 180,000 federal employees, for a total of more than 4.3 million, according to the Office of Personnel Management.

At the citizenship and immigration agency, officials said they had signed a $7.8 million lease in a gleaming new building, which they will occupy starting next month. During a recent speech in Los Angeles, the agency’s director, León Rodríguez, said that 5,000 people had already applied for the Crystal City jobs.

In the bulletin that the agency sent out, dated Dec. 1, the word “TODAY!” is printed in red next to a dozen jobs with titles like special assistant, management program analyst and immigration services officer. By the time the new Republican Congress takes up the debate about funding for the president’s immigration plan early next year, many of those new jobs are likely to be filled.

Some Republicans who have noticed the preparations for the new center have issued statements of outrage, but so far they have done little else. Senator Jeff Sessions, Republican of Alabama, called the new facility “a clear symbol of the president’s defiance of the American people, their laws and their Constitution.” He said in a statement that the new hiring would “foist on the nation laws Congress has repeatedly refused to pass.”

Mr. Rodríguez declined to be interviewed. But other administration officials readily say they are eager to put in place the infrastructure needed to allow undocumented people to apply for work permits by the early spring. That will require a new website, application forms and people to run background checks and process application fees that will probably be several hundred dollars.