“We see employers in a number of industries act as if there is a third-class labor market that is paid below the minimum wage and are made to suffer all manner of violations of labor law,” said Nik Theodore, an associate professor in urban planning and policy at the University of Illinois, Chicago. “For many employers it is a calculated risk that they are willing to take. They trust that their employees aren’t going to report them and that nobody from the government is actually going to come check. When you have workers in desperate need of work, they are going to be willing to do a lot of things.”

Over all, unreported income amounts to roughly $2 trillion annually, but cash wages make up only a portion of that estimate, according to Edgar L. Feige, an emeritus professor of economics at the University of Wisconsin, Madison, who has spent decades examining underground and cash economies, in part by using information on how much cash is in circulation at any given time. There is no way of knowing how many workers are earning their salaries in cash, Professor Feige said.

“Whatever unreported economy that exists, it goes well beyond immigration,” he said. The cash economy is particularly important in California, which has more undocumented immigrants than any other state and the eighth largest economy in the world. The sheer size of the immigrant work force and economy allows business owners to create a norm of paying off the books that would be unthinkable in another time or place, said Ruth Milkman, a labor expert and professor of sociology at the City University of New York Graduate Center.

“Employers have really gotten into the habit that this is the normal way of doing business,” Professor Milkman said. “They don’t particularly want to change, and nobody is making them do it. The immigration bill certainly doesn’t change much for employers who take the low road.”

Day laborers, those men standing in front of Home Depots and on street corners looking for whatever work comes their way, are perhaps the most widely recognizable stream of cash workers. Often, these men were mechanics, engineers or even architects and doctors in their home countries. The vast majority are undocumented immigrants. But in recent years, with the economy struggling, more of these immigrants have been standing in lots next to citizens, who are equally eager to find work, said Chris Newman, the legal director for the National Day Laborer Organizing Network.

“We see people who rotate in and out, they come find work for the day and go to another job at night,” Mr. Newman said. “You lose a foothold in the formal economy and it’s a natural place to go.”

Last month, the Labor Department reported that there are 2.7 million temp workers, more than ever before. As more large companies rely on temp agencies to fill their ranks, it is possible that more American workers, legally or not, could be treated like day laborers — who can be employed one day and out of a job the next. Mr. Newman repeated something that he has said to himself over and over again amid the debate in Washington: “Immigration laws are malleable, but the law of supply and demand is immutable.” The way he sees it, as long as there is growing demand for an informal labor market, there will be people to supply that work force.