Part of the problem, and one that the WSJ editorial staff is a repeat offender on, is the practice of lumping people like Tom Tancredo, who want immigration laws enforced and tightened in with isolationist paleocons like Pat Buchanan, who want all immigration cut off except for those fortunate few Catholics, Protestants and Jews who can speak English. This makes it easier to frame the argument in terms of "pro-immigration" and "anti-immigration" factions, and then the arguments go straight into the gutter. Things are a lot more complex than that, as usual. Aside from the Enforcers and the Know-Nothings, you have irredentist Chicano crazies and their allies in the Mexican government who have grandiose crack dreams about reversing the Treaty of Guadalupe Hidalgo and resorbing Texas, New Mexico, Arizona, Colorado and California back into Mexico. You also have Democrats who see the continuing flood of illegals as a way to use vote fraud to keep electing liberal Democrats to state legislatures and to Congress. You have businessmen who don't mind the flood of cheap labor, since it helps keep down the cost of labor in general and/or fills jobs poor/unemployed Americans won't do.
This isn't a partisan issue, either. It's a big source of friction within the Republican Party, and doesn't follow the usual split between libertarian and Christian conservatives; by the same token, you find center-left Democrats such as Hillary Clinton talking about the need to get control of the southern border. The President catches a lot of flak for pandering to the business community/shilling for the Latino vote by talking about amnesty, most recently from Peggy Noonan, who made the common-sense point that we really can't talk about "law-abiding undocumented workers" when their first act in America was to violate the immigration law.
On the Latino side of the fence, a lot of naturalized Chicanos and descendants of legal immigrants are pissed off that something isn't being done about stopping the flood, so there aren't any votes being won there. For the PRs and Cubans, this isn't even an issue - the PRs are Americans by birth, and the Cubans have immigration complaints of their own that have nothing to do with whether the Rio Grande is properly fenced or not. The Chicanos are also hacked off at the educrats who insist on hindering their childrens' assimilation by segregating them into classes which all too often only give lip service to teaching English while covertly (and sometimes overtly) sending the message that Spanish is just all right - no need to learn this difficult Anglo speech.
In the long run, though, the big problem for the United States with all these illegals pouring across the border is not only the possibility that terrorists and criminals might slip across, but that we might fall victim to the same demographic crisis that Europe finds itself in now. Culture affects politics, and the fact that Latino illegals share our Catholicism or Evangelical Protestantism is beside the point - do they share the same commitment to the rule of law under the Constitution? Have they bought into the American Dream that with hard work and clean living you can rise into the middle class and set your children on the path to something even better? If not, then we're going to have some serious problems in a couple of generations, the kind of problems that tend to get resolved by guns rather than debates.
UPDATE: ...I know that I've already written Europe off."
Ed Driscoll links to Lileks' interview on the Hewitt show, in which James talks about his reaction to the Steyn essay. Also included is a long quote from a Lee Harris essay on the internal contradictions of multiculturalism.