Will Wilkinson explains:

The energetic ideological base of the Republican Party is a nationalist, identity-politics movement for relatively well-to-do older white Americans known as the “tea party”. The tea party is interested in bald eagles, American flags, the founding fathers, Jesus Christ, fighter jets, empty libertarian rhetoric, and other markers of “authentic” American identity and supremacy. That America is “a nation of immigrants” is a stock piece of American identity politics, but the immigrants that made America America were, well, not Mexican, and spoke English, or at least Pennsylvania Dutch. Sorry Mexicans! Even if each element of immigration reform, taken in isolation, is agreed to be a good idea by a solid majority of Republican voters, Republican politicians must nevertheless avoid too-enthusiastically supporting this package of good ideas, lest they fail to project sufficient appreciation for the importance of keeping America American and putting Americans first. To fail to introduce a dead-on-arrival poison-pill amendment that would, say, require all would-be Americans to score over 160 on the LSAT, or to personally assassinate a member of al Qaeda before setting foot on the “path to citizenship” is to invite a primary challenge from a more thoroughly “American” American less cowed by the insidious deracinating forces of multicultural political correctness.

Despite this, he still expects GOP leaders to push something through.