Immigration is crawling its way back onto the national agenda--and not just as a footnote to keeping terrorists out. Earlier this year, Congress enacted a law intended to prevent illegal aliens from getting state drivers' licenses; the volunteer "minutemen" who recently patrolled the porous Arizona border with Mexico attracted huge attention, and members of Congress from both parties are now crafting proposals to deal with illegal immigration. All this is good. But unless we're brutally candid with ourselves, it won't amount to much. Being brutally candid means recognizing that the huge and largely uncontrolled inflow of unskilled Latino workers into the United States is increasingly sabotaging the assimilation process.

Americans rightly glorify our heritage of absorbing immigrants. Over time, they move into the economic, political and social mainstream; over time, they become American rather than whatever they were--even though immigrants themselves constantly refashion the American identity. But no society has a boundless capacity to accept newcomers, especially when many are poor and unskilled. There are now an estimated 34 million immigrants in the United States, about a third of them illegal. About 35 percent lack health insurance and 26 percent receive some sort of federal benefit, reports Steven Camarota of the Center for Immigration Studies. To make immigration succeed, we need (paradoxically) to control immigration.

Although this is common sense, it's common sense that fits uneasily inside our adversarial political culture. You're supposed to be either pro-immigrant or anti-immigrant--it's hard to be pro-immigrant and pro tougher immigration restrictions. But that's the sensible position, as any examination of immigration trends suggests.

Consider a new study of Mexican immigrants by Harvard economists George Borjas and Lawrence Katz. Mexicans are now the single largest group of U.S. immigrants, 30 percent of the total in 2000. Indeed, the present Mexican immigration "is historically unprecedented, being both numerically and proportionately larger than any other immigrant influx in the past century," note Borjas and Katz. In 1920, for example, the two largest immigrant groups--Germans and Italians--totaled only 24 percent of the immigrant population.

Some Mexican-Americans have made spectacular gains, but the overall picture is dispiriting. Among men, about one in 20 U.S. workers is now a Mexican immigrant; in 1970, that was less than one in 100. The vast majority of Mexican workers lacked a high-school diploma in 2000 (63 percent for men, 57 percent for women). Only a tiny share had college degrees (3 percent for men, 5 percent for women). By contrast, only 7 percent of native-born U.S. workers were high-school dropouts and 28 percent were college graduates in 2000. Mexican workers are inevitably crammed into low-wage jobs: food workers, janitors, gardeners, laborers, farm workers. In 2000, their average wages were 41 percent lower than average U.S. wages for men and 33 percent lower for women.

What's particularly disturbing about the Borjas-Katz study is that children of Mexican immigrants don't advance quickly. In 2000, Americans of Mexican ancestry still had lower levels of educational achievement and wages than most native-born workers. Among men, the wage gap was 27 percent; about 21 percent were high-school dropouts and only 11 percent were college graduates. Borjas and Katz can't explain the lags. "What's the role of culture vs. lousy [U.S.] schools?" asks Katz. "It's hard to say." Borjas doubts that the cause is discrimination. Low skills seem to explain most of the gap, he says. Indeed, after correcting for education and age, most of the wage gap disappears. Otherwise, says Borjas, "I don't know."

But some things we do know--or can infer. For today's Mexican immigrants (legal or illegal), the closest competitors are tomorrow's Mexican immigrants (legal or illegal). The more who arrive, the harder it will be for existing low-skilled workers to advance. Despite the recession, immigration did not much slow after 2000, says Camarota. Not surprisingly, a study by the Pew Hispanic Center found that inflation-adjusted weekly earnings for all Hispanics (foreign and American-born) dropped by 2.2 percent in 2003 and 2.6 percent in 2004. "Latinos are the only major group of workers whose wages have fallen for two consecutive years," said the study. Similarly, the more poor immigrants, the harder it will be for schools to improve the skills of their children. The schools will be overwhelmed; the same goes for social services.

We could do a better job of stopping illegal immigration on our southern border and of policing employers who hire illegal immigrants. At the same time, we could provide legal status to illegal immigrants already here. We could also make more sensible decisions about legal immigrants--favoring the skilled over the unskilled. But the necessary steps are much tougher than most politicians have so far embraced, and their timidity reflects a lack of candor about the seriousness of the problem. The stakes are simple: will immigration continue to foster national pride and strength or will it cause more and more weakness and anger?