The Texas cotton lobbyist tried to reassure Congress that the tens of thousands of Mexicans who labored in the fields of the Southwest were not a threat to national security. There "never was a more docile animal in the world than the Mexican," he told the Senate committee.
 Then he offered a way around the political problem the congressmen faced in extending the program that had let the workers in.
 "If you gentlemen have any objections to admitting the Mexicans by law," he said, "take the river guard away and let us alone, and we will get them all right."
 They did — and that was in 1920. Almost a century later, the debate over illegal immigration from Mexico often makes it sound like a recent development that breaks with the tradition of legal passage to America.
 Quite the contrary, say immigration scholars like Aristide R. Zolberg, who relates the anecdote about the Texas cotton grower in his new book, "A Nation by Design: Immigration Policy in the Fashioning of America." A pattern of deliberately leaving the country's "back door" open to Mexican workers, then moving to expel them and their families years later, has been a recurrent feature of immigration policy since the 1890's.
 "Things are not the same today, but the basic dynamics do not change," said Mr. Zolberg, a professor of political science at the New School. "Wanting immigrants because they're a good source of cheap labor and human capital on the one hand, and then posing the identity question: But will they become Americans? Where is the boundary of American identity going to be?"
 Nearly every immigrant group has been caught at that crossroads for a time, wanted for work but unwelcome as citizens, especially when the economy slumps. But Mexicans have been summoned and sent back in cycles for four generations, repeatedly losing the ground they had gained.
 During the Depression, as many as a million Mexicans, and even Mexican-Americans, were ousted, along with their American-born children, to spare relief costs or discourage efforts to unionize. They were welcome again during World War II and cast as heroic "braceros." But in the 1950's, Mexicans were re-branded as dangerous, welfare-seeking "wetbacks."
 In 1954, President Dwight D. Eisenhower sent Gen. Joseph Swing to "secure the border" with farm raids and summary deportations that drove out at least a million people. At the same time, growers were assured of a new supply of temporary workers through the "braceros" program, which soon doubled to 400,000 a year.
 The pattern grew during the years between the 1882 Chinese Exclusion Act and the quotas of 1929, as rising legal barriers drastically narrowed the nation's front door. The goal was to preserve the country's "Nordic character" against Italians and Eastern European Jews who had begun arriving in large numbers.
 Yet Congress refused to close the back entrance to a growing flow of Mexicans, even though by the lawmakers' own racial standards, Mexicans were even more objectionable than the "degraded races" of Asians and Southern Europeans whom they were increasingly replacing in fields, factories and railroad work.
 A convenient way was found to reconcile the contradiction, said Camille Guérin-Gonzales, a professor of history at the University of Wisconsin and the author of "Mexican Workers and American Dreams." No quotas were necessary to keep Mexicans out because they were not going to stay. "Not wanting to 'mongrelize the race,' but needing cheap labor, Americans constructed Mexicans as 'birds of passage,' " she said, using the phrase coined to describe Italian immigrants. "The proximity of the border made that even more believable."
 The cotton pickers cited by the Texas lobbyist had arrived by way of a program intended to address World War I labor shortages. But as commercial agriculture created "factories in the field," undocumented entry became the norm. Growers pointed out that no willing field hand could afford the "head tax" that went with legal entry. And employers regularly cited informal entry as a feature that made Mexicans more desirable than cheap foreign laborers like Filipinos, because they were easier to deport. As one rancher quoted in Mr. Zolberg's book remarked to a Mexican hand: "When we want you, we'll call you; when we don't — git."
 The full, brutal weight of that formula hit in the Depression. Roundups of Mexican families in public places, summary deportations — and well-publicized threats of more to come — sent panic through Mexican-American communities in 1931. The tactic was called "scare-heading" by its architect, Charles P. Visel, the director of the Los Angeles Citizens Committee on the Coordination of Unemployment Relief. It worked. Even many legal immigrants were panicked into selling their property cheap and leaving "voluntarily."
 It was a time when crops went unharvested for lack of buyers and white families like those in "The Grapes of Wrath" poured West, desperate for work. "They gave you a choice: starve or go back to Mexico," a resident of Indiana Harbor, Ind., recalled later, as Roger Daniels relates in his book "Guarding the Golden Door."
 A Santa Barbara woman said she would never forget seeing trains organized by the railroad transporting families to the border in boxcars. The same rail lines had long been maintained by Mexicans who had settled not only in the Southwest, but in Indiana, Illinois and eastward.
 "I have left the best of my life and strength here, sprinkling with the sweat of my brow the fields and factories of these gringos, who only know how to make one sweat and don't even pay attention to one when they see one is old," said one worker, Juan Berzunzolo, interviewed in California in the 1920's by a Mexican anthropologist and quoted by Devra Weber in "Dark Sweat, White Gold: California Farm Workers, Cotton and the New Deal."
 At the other side of the border, Ms. Guérin-Gonzales said, an 11-year-old American-born girl who had been "repatriated" from California told an interviewer in the 1930's, "I would be in the fifth grade there, but here, no, because I didn't know how to read and write Spanish." A boy recounted how a Mexican policeman upbraided him for speaking English. But by 1943, with the economy ascendant and employers crying of wartime labor shortages, the cycle began anew.
 Today, the nature of the deal can no longer be disguised, said Marcelo M. Suárez-Orozco, co-director of Immigration Studies at New York University. "It's a bad-faith pact," he said. "We can't have it both ways — an economy that's addicted to immigrant labor, but that's not ready to pay the cost."
 And Mr. Zolberg said the old resort to mass expulsion is less likely, since the naturalization of millions of Latinos, including those from the 1986 amnesty, changed the rules of the game. "Mexicans, and Latinos generally are more in the situation today that Italians and Jews were in the 20's and 30's," he said. "They began to have some electoral clout, because there were more people of that national origin who could stand up."
Applications to American Immigrant Literature:
Mexican Americans repeatedly cross national borders, confusing allegiances and government responsibilities.
Mexican Americans follow traditional immigrant patterns of seeking economic opportunity but are exploited like minority groups. Compared to other immigrant groups, minority status lasts longer?
Paragraph 12 re "mongrelize the race" attitude reflects USA dominant culture emphasis on purity, bans on racial intermarriage.
Paragraph 14: coerced migration may resemble Trail of Tears