There is little question that the vast majority of those who enter the United States illegally are looking for work. And few doubt that those who stay for long find it.
So can the United States win control over its borders by making it illegal to hire undocumented workers?
This hope is at the heart of the Simpson-Mazzoli immigration reform bill as passed now by both the US House and Senate.
But there is some strong skepticism in the Southwest, where the largest numbers of illegal employees are concentrated, on the practical issue of how successful the employer sanctions will be.
In the House version, employers would be fined $2,000 per illegal employee. The Senate version would impose criminal charges against repeat offenders.
To succeed, these sanctions must overcome three problems:
* Employers have to be able to replace illegal workers at a cost that doesn't drive them out of business.
* Either the Immigration and Naturalization Service or the Department of Labor will have to come with the manpower to enforce the sanctions.
* The government will have to find a way to identify workers in spite of a booming trade in fake documents.
Working against the sanctions are economics and custom. Some of the fastest-growing labor markets in the economy are in industries that tend to employ illegal aliens: restaurants, hotels, food processing, electronics, light manufacture, landscaping, and building maintenance.
The degree of dependence on an illegal work force varies widely from company to company. But in the agriculture and garment industries in the Western US, the labor pool is so tied to illegal immigration that even employers who hire only legal residents fear debilitating shortages if the illegal workers disappear, since the pool of legal workers will be in such high demand.
''You can't assume that because of the (Simpson-Mazzoli) bill there will be any change in labor demand,'' says Maria Patricia Fernandez Kelly, a researcher at the Center for US-Mexico Studies at the University of California at San Diego. ''There will be rising demand for this type of work.''
Farmers may get a guest-worker program in the Simpson-Mazzoli package that will help them replace their undocumented harvest hands. But most of the illegal aliens working in the US are in other industries.
Dr. Fernandez Kelly and some colleagues at the center are researching 177 nonfarm American companies that have employed illegal aliens. Her observation, supported by other researchers, is that the biggest, most-visible employers will comply with Simpson-Mazzoli and replace their illegal workers with legal workers or machines.
But the smaller, more marginal companies may flaunt the law. ''Most of these are small companies,'' Dr. Fernandez Kelly says, ''and the undocumented workers make a difference in cost.''
In the construction industry, for example, the largest projects are mostly under union contract. But Los Angeles has developed a large curbside labor market, in which smaller contractors drive by street-corner gathering places in the morning and pick up the workers they need, at $3 to $5 an hour. These workers, most illegal, represent a much lower cost to the employer.
Congressional aides that drafted Simpson-Mazzoli estimate that 75 percent of employers will comply with the law without enforcement. (This figure is roughly based on the experience of other federal agencies such as Occupational Safety and Health Administration and the Environmental Protection Agency.) The Immigration and Naturalization Service (INS) could then go after the flagrant violators.
How well the already overburdened INS will be able to enforce this law is a matter for debate. Loy Bilderback, professor of history at California State University at Fresno, who has written a book on undocumented Mexican labor in the US, says that ''the INS and the Department of Labor do not have the manpower to do it.''
Whether the employer sanctions work, Dr. Bilderback says, ''comes down to how badly the administration wants to enforce it.''
If the INS is afforded the manpower it would need to enforce Simpson-Mazzoli, it is still not clear that the sanctions can be designed so they will stick in any but the most flagrant cases. There is a thriving market in fraudulent documents for sale in Tijuana. Employers may be required to check the identification of workers it employs, but they won't be held responsible for the authenticity of the papers. As long as an employer can show that he asked for and saw two documents that were not obvious frauds, it may be difficult to hold him responsible for hiring an illegal alien.