“Which Side Are You On?” was the great labor song of the 1930s. The question in its title remains relevant. Back then, the two sides – the union or thugs from the mining company – made for an easy selection. Now, workers have to choose between their own economic security and mass immigration, which their unions simply opposed in the past.

The new options are more painful because today’s immigrants, legal or otherwise, are mostly good, hard-working people and potential union members. But the law of labor supply and demand states – and history confirms – that wages and union power fall in times of high immigration. In recent decades, organized labor’s official stance has moved toward the open-borders position.

Vernon Briggs has been hacking through this confusion for a long time. A labor-relations expert at Cornell University, Briggs is a pro-union Democrat. He recently told the House immigration subcommittee that the ongoing flood of workers into the United States hurts organized labor. This puts him at variance with many of today’s union leaders, though not their predecessors.

“Historically, the labor movement has been able to distinguish between the immigrant agenda and the agenda for American workers,” Briggs told me. “It’s traditionally pushed for the agenda that favored American workers, even though many of them were immigrants.”

The AFL-CIO passionately supported the 1986 Immigration Reform and Control Act, particularly the part calling for fines against employers who hire illegal workers.

Thereafter, labor leaders started to cave on immigration. The AFL-CIO made little fuss when the Immigration Act of 1990 raised the number of legal visas by 35 percent to about 700,000 a year. At its 1993 convention, members passed a resolution that accused immigration-reform advocates of launching “a new hate campaign” that made immigrants “the scapegoats for economic and social problems.”

Why the about-face? The unions figured that the federal government wasn’t going to control immigration so they might as well try to organize the newcomers. Problem is, the swelling tide of new labor competition has undermined their ability to improve the workers’ lot.

And it’s true that recent “victories” in unionizing low-skilled workers have produced paltry gains. For example, the Service Employees International Union managed to organize janitors in Los Angeles, but Briggs notes, “at wages way below what they were back in the 1970s.”

The union last year organized janitors in Houston. For all these efforts, this largely Hispanic work force saw its pay rise from a pitiful $5.25 an hour to a pathetic $6.25 – which is lower than the minimum wage in 21 states and the District of Columbia.

This is an unattractive chore, but the American people have to choose sides. It’s either continued massive immigration or providing relief to their downwardly mobile workers. They can’t have both.