The groups have provided the intellectual framework and grass-roots muscle for opposing legislation that would legalize millions of illegal immigrants.
Well-funded and politically savvy, the groups produce research papers, testify at congressional hearings and appear frequently in the media to push for reducing immigration. Numbers USA reports that its members have inundated the office of Sen. Marco Rubio, R-Fla., with 100,000 faxes this year warning him that his central role in pursuing changes in immigration laws could damage his future political prospects.
The groups have established close relationships with some of Congress's most vocal critics of more liberal immigration laws.
The Center for Immigration Studies' website, for instance, features testimonials in which Rep. Lamar Smith, R-Tex., lauds the center's "credible and articulate voice" and Sen. Jeff Sessions, R-Ala., thanks it for providing "invaluable research." When Republican presidential candidate Mitt Romney last year endorsed a policy of "self-deportation" in which he said cracking down on illegal immigrants would force many of them to leave on their own, his position matched a policy laid out years earlier by CIS called "attrition through enforcement."
The groups are front and center again this year, with a CIS official appearing before a key Senate committee Wednesday and the other groups mobilizing members to lobby against a possible bipartisan deal on citizenship.
Conservatives who are taking on the groups, including Rubio, anti-tax activist Grover Norquist and officials of the Catholic Church, argue that the three organizations are motivated by far different philosophies than many of their Republican allies realize. Among those views: that population growth from increased migration threatens the environment.
The Republicans orchestrating the campaign against the groups have long rejected their views on immigration, and liberal immigration advocates have long made a practice of attacking the organizations. Now, with such GOP leaders as House Speaker John Boehner, Ohio, saying immigration legislation is a priority, some Republicans see an opportunity to loosen what they say has been the groups' stranglehold on party orthodoxy.