Americans ought to hope for the success of Vicente Fox Quesada, the new president-elect of Mexico. He broke the 71-year rule of the Institutional Revolutionary Party (known as PRI), creating a broader and more genuine democracy. He promises to attack corruption, curb the drug trade, encourage private investment and increase economic growth. Fox deserves our support and sympathy. But we should not let goodwill slip into sentimentality. American and Mexican interests sometimes collide--on immigration, for instance, where Fox seems to have large ambitions.

Our interest lies in less immigration from Mexico, while Mexico's interest lies in more. The United States has long been an economic safety valve for Mexico: a source of jobs for its poor. By World Bank estimates, perhaps 40 percent of Mexico's 100 million people have incomes of less than $2 a day. The same desperate forces that drive people north mean that once they get here they face long odds in joining the American economic and social mainstream.

The United States may (or may not) need more immigrants--this is a subject of much disagreement. But we surely don't need more poor and unskilled workers, and Mexican immigrants fall largely in this category. The stakes here transcend economics. Americans are justly proud of being a nation of immigrants. People of many lands and customs have become American--which is different from what they were--even as they refashioned what it means to be American. By contrast, many Mexican immigrants have little desire to "join the American mainstream" precisely because their overriding motive for coming was economic and their homeland is so close. Their primary affection remains with Mexico.

This is understandable, even commendable. (In 1997 only 15 percent of the estimated 7 million Mexican immigrants had become U.S. citizens. One reason, of course, is that perhaps 3 million are thought to be illegal.) But it is equally understandable that most Americans wish to preserve the nation's immigrant heritage--and not become simply a collection of peoples, from various places, who happen to work here and whose main allegiances lie elsewhere.

Fox's basic diagnosis of the immigration problem is sound. He believes that only greater prosperity in Mexico--more jobs, higher incomes--can reduce the pressures to migrate. If people live better, they will stay home. Since 1996 Mexico's economy has grown about 5 percent annually. Fox aims to raise that to 7 percent by 2006. He talks, somewhat inconsistently, of ultimately having open borders between the United States and Mexico, much as exist between members of the European Union. At one point, he says this could occur in a decade. At another point, he concedes it would require a convergence of incomes (perhaps 7 to 1 in America's favor) to prevent a mass exodus of Mexicans. Optimistically, that would take decades.

Meanwhile, he apparently hopes to raise legal immigration. This is the gist of various press leaks. He seems to suggest a bargain: The United States would allow more legal immigration; in return, Mexico would crack down--as it hasn't in the past--on illegal immigration. Already, Mexico is the largest source of legal immigrants, representing about 20 percent in 1998. The Wall Street Journal quotes one Fox adviser as saying that legal visas should increase by about 180,000, which would more than double their 1998 level.

For the United States, this would be a bad bargain. No one knows the number of Mexicans who come and stay illegally each year. The Immigration and Naturalization Service's last estimate (which dates from 1996) is 150,000. If this is correct, the proposed increase in legal visas would exceed the present number of illegal immigrants. Overall immigration would rise even if--miraculously and implausibly--illegal immigration stopped altogether.

The United States cannot act as a sponge for Mexico's poor. In the present boom, immigration is an issue easily forgotten. Anyone can get a job, we say. Immigrants (it's argued) have helped prevent a wage-price spiral. Up to a point, they may have. But the boom won't last forever, and the least-skilled immigrants always struggle.

The most obvious consequence of allowing more Mexican immigrants into the country would be to hurt those already here. The two groups clearly compete. An increase of 10 percent in new immigrants can reduce the wages of earlier immigrants by 9 or 10 percent, says a report from the Urban Institute in Washington. Fewer than half of Mexican Americans over 25--including those born in the United States--were high school graduates in 1996, according to a study from the National Council of La Raza, an advocacy group for Latinos. The same report warns that workers with poor English can do only "basic tasks at entry-level positions offering low wages." All this is common sense.

The power of America's economy, culture and society to assimilate immigrants is enormous. History is clear: The children of immigrants increasingly become American. But that power is not unlimited. The job market, schools and social services can be overwhelmed by large numbers, especially--as is the case with Mexicans--when most immigrants come to only two states, California and Texas.

The dangers are balkanization--a society increasingly fractured along class and ethnic lines--and a backlash against immigration. A possible perverse side effect is a rise in prejudice against Hispanic Americans, who are confused for immigrants, even though they've often lived here for generations. This has long concerned civil rights groups, such as La Raza.

There is a difference between having open borders for goods and for people. The theory of NAFTA (the North American Free Trade Agreement) was that both the United States and Mexico could prosper from more trade and international investment. The theory remains powerful, even if it's no instant panacea for all of Mexico's problems. Vicente Fox and the next U.S. president have plenty of areas where they can cooperate to mutual advantage. But higher Mexican immigration isn't one of them.