The fate of the Republic was never going to hang on the bill that Gov. Gray Davis signed last year allowing illegal immigrants to apply for California driver's licenses. But it was a symbol, and the backlash it created was another reminder that in California at least, Mexican immigration is the 800-pound policy gorilla that few politicians dare confront.

But if Samuel Huntington has his way, the issue will return with a vengeance.

Huntington, a longtime Harvard political science professor, a former senior official at the National Security Council and now chairman of the Harvard Academy for International Area Studies, has launched a salvo at Mexican immigration that seeks to put the stamp of academic respectability on the red meat of conservative talk shows and Web sites.

For many years, Huntington's theme has been what he called "the clash of civilizations." But according to Huntington's new book, due out in May, and an inflammatory article based on it ("The Hispanic Challenge" in the current issue of Foreign Policy magazine), one such clash is now taking place in this country.

"Unlike past immigrant groups," he says, "Mexicans and other Latinos have not assimilated into mainstream U.S. culture, forming instead their own political and linguistic enclaves ... and rejecting the Anglo-Protestant values that built the American dream."

The United States, Huntington believes, ignores this challenge at its peril.

"Demographically, socially and culturally, the reconquista (reconquest) of the Southwest United States by Mexican immigrants is well under way." As their numbers grow, he says, Mexican Americans "feel increasingly comfortable and [are] often contemptuous of American culture." If that continues, "the cultural division between Hispanics and Anglos could replace the racial division between blacks and whites as the most serious cleavage in U.S. society."

A lot of Huntington's factual assumptions are nonsense. Among American-born Latinos, a bare 4 percent are Spanish-dominant speakers; 61 percent are English-dominant. Nearly all are certain that a command of English is essential to success in the United States. The children of immigrants are buying homes, starting businesses, intermarrying with members of other ethnic groups and accommodating to American culture as all other immigrants have done.

Nor is there a push for separatism. Much of south Texas, says Gregory Rodriguez, a senior fellow at the New America Foundation in a recent piece in the Los Angeles Times, has had majority Hispanic populations for generations "and separatism - political, cultural or economic - is nowhere to be found.

"[Huntington's] belief in cultural conflict leads him to treat cultural behaviors that are fluid and adaptable as if they were biologically predetermined. ... Huntington's theory doesn't take into account the people whose actions it presumes to predict. In the more than a century and a half of Mexican American history, there has not been one serious, popularly supported movement to wrest control of the Southwest away from the U.S. or to isolate it from the rest of the nation."

But Rodriguez's biggest differences are with Huntington's definition of assimilation. Can maintaining ethnic culture and tradition be consistent with the embrace of American political, social and economic values, as Huntington himself once believed? Or is it necessary, if America is to cohere as a nation, for all immigrants "to assimilate into Anglo Protestant culture?"

Huntington has no real remedy. He says if immigration were cut in half, a lot of other issues - bilingual education and the debate over whether immigrants burden public services - would disappear, and the wages of low-income U.S. workers would improve.

But given the enormous wage differentials between the United States and Mexico and the fractious politics of immigration, sharp reductions in immigration are not likely.

Prior attempts to reduce illegal immigration - employer sanctions, guest worker programs, the hope that NAFTA, the North American Free Trade Agreement, would generate more and better Mexican jobs - have had little success. On the contrary, legal guest workers, or the aliens whose status was legalized, brought more illegals.

Philip Martin, who studies immigration issues at UC Davis, says that even the discussion of President Bush's legalization proposals has brought more people across the border, eager to get a leg up on future legalization.

Employers, he says, want workers, "but what they get is people."

There's no doubt that Huntington raises a crucial issue, even for liberals. The voters' willingness to support high quality public services almost certainly varies inversely with the public's perception of the rate of immigration.

"When California is able to provide in-state college tuition to illegal immigrants," says Jim Brulte, who leads the GOP caucus in the state Senate, "voters know that we're not short of money."

But Huntington's article is as likely to inflame the debate as it is to foster a balanced discussion of an issue that's difficult enough already. It will reinforce each side's prejudices and frustrations, confirming the arguments of Latino militants that they're not welcome in the U.S. mainstream, reinforcing the belief of exclusionists that the enemy is at the gates. Neither is correct.