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.