GOP, you are warned: immigration could cause a Republican crackup.

NO issue, not one, threatens to do more damage to the Republican coalition than immigration. There's no issue where the beliefs and interests of the party rank-and-file diverge more radically from the beliefs and interests of the party's leaders. Immigration for Republicans in 2005 is what crime was for Democrats in 1965 or abortion in 1975: a vulnerable point at which a strong-minded opponent could drive a wedge that would shatter the GOP.

President Bush won reelection because he won 10 million more votes in 2004 than he did in 2000. Who were these people? According to Ruy Teixeira-a shrewd Democratic analyst of voting trends-Bush scored his largest proportional gains among white voters who didn't complete college, especially women. These voters rallied to the president for two principal reasons: because they respected him as a man who lived by their treasured values of work, family, honesty, and faith; and because they trusted him to keep the country safe.

Yet Bush is already signaling that he intends to revive the amnesty/guestworker immigration plan he introduced a year ago-and hastily dropped after it ignited a firestorm of opposition. This plan dangerously divides the Republican party and affronts crucial segments of the Republican vote.

The plan is not usually described as an "amnesty" because it does not immediately legalize illegal workers in this country. Instead, it offers illegals a three-year temporary work permit. But this temporary permit would be indefinitely renewable and would allow illegals a route to permanent residency, so it is reasonably predictable that almost all of those illegals who obtain the permit will end up settling permanently in the United States. The plan also recreates the guestworker program of the 1950s-allowing employers who cannot find labor at the wages they wish to pay to advertise for workers outside the country. Those workers would likewise begin with a theoretically temporary status; but they too would probably end up settling permanently.

This is a remarkably relaxed approach to a serious border-security and labor-market problem. Employers who use illegal labor have systematically distorted the American labor market by reducing wages and evading taxes in violation of the rules that others follow. The president's plans ratify this gaming of the system and encourage more of it. It invites entry by an ever-expanding number of low-skilled workers, threatening the livelihoods of low-skilled Americans--the very same ones who turned out for the president in November.

NATIONAL REVIEW has historically favored greater restrictions on legal as well as illegal immigration. But you don't have to travel all the way down the NR highway to be troubled by the prospect of huge increases in immigration, with the greatest increases likely to occur among the least skilled.

The president's permissive approach has emboldened senators and mayors (such as New York's Michael Bloomberg) to oppose almost all enforcement actions against illegals. In September 2003, for example, Bloomberg signed an executive order forbidding New York police to share information on immigration offenses with the Immigration Service, except when the illegal broke some other law or was suspected of terrorist activity. And only last month, a House-Senate conference stripped from the intelligence-overhaul bill almost all the border-security measures recommended by the 9/11 commission.

The president's coalition is already fracturing from the tension between his approach to immigration and that favored by voters across the country. Sixty-seven House Republicans--almost one-third of the caucus--voted against the final version of the intelligence overhaul. And I can testify firsthand to the unpopularity of the amnesty/guestworker idea: I was on the conservative talk-radio circuit promoting a book when the president's plan was first proposed last January. Everywhere I went, the phones lit up with calls from outraged listeners who wanted to talk about little else. Every host I asked agreed: They had not seen such a sudden, spontaneous, and unanimous explosion of wrath from their callers in years.

Five years ago, Candidate George W. Bush founded his approach to immigration issues on a powerful and important insight: The illegal-immigration problem cannot be solved by the United States alone. Two-thirds of the estimated 9 million illegals in the U.S. are from Mexico. Mexico is also the largest source of legal immigration to the United States. What caused this vast migration? Between 1940 and 1970, the population of Mexico more than doubled, from 20 million to 54 million. In those years, there was almost no migration to the United States from Mexico at all. Since 1970, however, some 65 million more Mexicans have been born--and about 20 million of them have migrated northward, with most of that migration occurring after 1980.

Obviously, the 30 years from 1940 to 1970 are different in many ways from the 30 years after 1970s. But here's one factor that surely contributed to the Mexican exodus: In the 1940s, '50s, and '60s, the Mexican economy grew at an average rate of almost 7 percent a year. Thanks to the oil boom, the Mexican economy continued to grow rapidly through the troubled 1970s. But since 1980, Mexico has averaged barely 2 percent growth. The average Mexican was actually poorer in 1998 than he had been in 1981. You'd move too if that happened to you.

Recognizing the connection between Mexican prosperity and American border security, the Reagan, Bush, and Clinton administrations all worked hard to promote Mexican growth. The Reagan and Clinton administrations bailed out Mexican banks in 1982 and 1995; the first Bush administration negotiated, and Clinton passed, NAFTA. George W. Bush came to office in 2001 envisioning another round of market opening with the newly elected government of his friend Vicente Fox, this time focusing on Mexico's protected, obsolete, economically wasteful, and environmentally backward energy industry.

Bush's hopes have been bitterly disappointed. The Fox government has actually done less to restore Mexican growth than the PRI governments of the 1990s. And so Bush has been pushed away from his grand vision and has instead accepted Fox's demand that the two countries concentrate on one issue: raising the status of Mexican illegals in the United States. But this won't work. Just as the U.S. cannot solve the problem by unilateral policing, so it also cannot solve it through unilateral concession. Bush had it right the first time.

Some of the president's approach to immigration remains right and wise. He is right to show a welcoming face to Hispanics legally resident in the United States. He is right to try to smooth the way to citizenship for legal permanent residents. He is right--more controversially--to give all who have contributed to Social Security, whatever their legal status, access to benefits from the Social Security account.

But he is wrong, terribly wrong, to subordinate border security to his desire for an amnesty deal-and still more wrong to make amnesty the centerpiece of his immigration strategy.

Right now, of course, the president does not have to worry much about political competition on the immigration issue. But Republicans shouldn't count on their opponents' ignoring such an opportunity election after election. "I am, you know, adamantly against illegal immigrants," Hillary Clinton told a New York radio station in November. And later: "People have to stop employing illegal immigrants. I mean, come up to Westchester, go to Suffolk and Nassau counties, stand on the street corners in Brooklyn or the Bronx. You're going to see loads of people waiting to get picked up to go do yard work and construction work and domestic work." Okay, so maybe Hillary will never pick up many votes in Red State America. But there are Democratic politicians who could.

Republicans need a new and better approach-one that holds their constituency together and puts security first.

First, Republicans should develop and practice a new way of speaking about immigration, one that makes clear that enforcement of the immigration laws is not anti-immigrant or antiMexican: It is anti-bad employer. Illegal immigration is like any other illegal business practice: a way for unscrupulous people to exploit others to gain an advantage over their law-abiding competitors.

Second, Republicans can no longer deny the truth underscored by the 9/11 commission: Immigration policy is part of homeland-security policy. Non-enforcement of the immigration laws is nonprotection of Americans against those who would do them harm.

Third, Republicans have to begin taking enforcement seriously. It's ridiculous and demoralizing to toss aside cabinet nominees like Linda Chavez over alleged immigration violations while winking at massive law-breaking by private industry--or to regard immigration violations as so trivial that they can be used as a face-saving excuse for the dismissal of a nominee damaged by other allegations.

Fourth, skills shortages in the high-technology and health-care industries are genuine problems that have to be addressed-but they should not be used as an excuse to void immigration enforcement. Republicans can say yes to using immigration law to attract global talent, while saying no to companies that systematically violate immigration law to gain an advantage over their more scrupulous rivals.

Fifth, Mexico should not be allowed to sever the migration issue from trade and investment issues. Mexican political stability is a vital national-security issue of the United States--and just for that reason, Americans should not allow Mexican governments to use migration as a way to shirk the work of economic and social reform.

Finally--and most important-Republicans need to recognize that they have a political vulnerability and must take action to protect themselves. An election victory as big as 2004 can look inevitable in retrospect. But it wasn't, not at all. The Democrats could have won--and could still win in 2006 and 2008--by taking better advantage of Republican mistakes and making fewer of their own. And no mistake offers them a greater opportunity than the one-sidedness of the Bush immigration policy. The GOP is a party dedicated to national security, conservative social values, and free-market economics. The president's policy on immigration risks making it look instead like an employers' lobby group. That's the weak point at which the edge of the wedge could enter-and some smart Democratic politician is sharpening it right now.