Objections to immigration usually surge when unemployment is high. But U.S. unemployment is at 4.7 percent. The traditional argument that immigrants are taking jobs from locals simply doesn’t make sense in the current American case.

Still, President George W. Bush is pushing Congress to send thousands of troops to the Mexican border and insisting that Congress pass legislation that funds vigorous policing of immigration. Though the plan aims to be humane, it will complicate the lives of the many struggling workers without papers in the U.S.- construction workers, accounting students, small business staff, manicurists.

Why all this action now? The answers are about national identity. They aren’t at all logical. But neither is the part of the human brain that thinks about such questions.

The first answer can be found in the election cycle. The sixth year of a presidency isn’t usually a good year for an incumbent or his party, and Bush is no exception as his approval ratings hover around the 30 percent mark. Bush has always liked immigrants, and ignored the anti-immigration wing of the Republican Party – lawmakers such as Tom Tancredo of Colorado. This year, though, Bush needs Tancredo & Co. at midterm elections in November.

The war is also part of the shift. The Sept. 11 terrorists came from abroad. The administration fears that one day terrorists will cross into Texas or California via Mexico. So policing that border makes more sense than it used to.

There is also what one might call the cortisol factor, after the hormone that the body produces in response to stress. Cortisol converts open, friendly humans into control freaks and bullies. Almost five years into this stressful war, the U.S. can’t find Osama bin Laden. Some of that resentment is being rerouted into the hunt for illegals. Hence, Bush’s declaration in his immigration speech Monday: “First, the U.S. must secure its borders.”

But the principal reason for the shift is cultural. Historically, the U.S. liked to think of itself as a country of immigrants. With a few dreadful exceptions, it was tolerant of immigrants’ cultures as long as those cultures stayed mostly in the private sphere.

The welcome assumed that immigrant families would assimilate in later generations. Most Americans assumed too that English would always be the common language of the public sector – schools, municipal and federal offices. The English rule represented a sort of national village green.

More recently, Americans have tended to welcome immigrants for another reason. Immigrants challenged established groups in ways that benefit the country and produce growth.

Immigrant workers did organized labor a favor by forcing it to face up early to global competition. Immigrant children challenge the complacent offspring of established natives.

And immigrants challenge monomaniacal American corporations. Without immigrant Sergey Brin of Google Inc., Bill Gates would be more self-satisfied and Microsoft Corp. a less interesting company. Without Andy Grove from Hungary at Intel Corp., IBM would have been much slower to innovate.

How much lower would the economy’s growth lately have been without immigrants? A quarter of a percentage point a year? A full percentage point?

Immigrants have tended to sustain the U.S. that most of us love best: a growing country where individuals have a chance, as opposed to a static country of competing power blocs.

The problem is that in the past three decades immigrants have learned that they can build power blocs of their own. If women, blacks, the elderly and the disabled have government-recognized identities, then immigrants tend to want to join those groups or create their own.

But state- or federally-funded multiculturalism can do damage, bilingual education being the best example. Instead of serving as a bridge to help integrate newcomers, Spanish-language instruction frequently ghetto-ized younger arrivals.

They didn’t learn English well enough to pursue higher education. They missed out on the great unifying American experience. Bilingualism is built on the principle of fairness. But there is nothing fair about encountering it when you are an immigrant whose mother tongue is yet a third language.

Government multiculturalism also hurts national identity. As one of the authors of California’s school curriculum, Diane Ravitch points out that state’s law mandates representation of all social groups in instructional material, from the elderly to women and the disabled. Political jargon makes the books incoherent. The result isn’t so much that students learn the wrong history, but that they don’t learn history at all.

The fear nowadays is that more immigration will mean more official multiculturalism, or even multinationalism. If the choice is between bifurcating the U.S. so that it is like Canada and restricting immigration, Americans will tend to choose restricting immigration.

The president has spoken of assimilation and recently said that he prefers that the Star Spangled Banner be sung in English. But policy strong enough to solve the identity problem is missing in his plan. He has failed to emphasize that official bilingualism hurts the country. Or that it is time to end government set-aside contracts for political blocs. Or to emphasize that the U.S. is a country of individuals, not groups.

Such omissions mean that some hostility to immigrants will remain, even if unemployment hits zero. The U.S. needs to welcome newcomers to grow. But a country that doesn’t know who it is has a hard time playing host.

Miss Shlaes is a columnist for Bloomberg News.