The cost to local, state, and federal governments in the United States of providing social services to millions of resident aliens, legal or illegal, may be one of the main reasons immigration reform bill may not be approved by Congress this year.
President Reagan has hedged on his support of the controversial Simpson-Mazzoli reform package, because he wants limits placed on the amount of reimbursement to local governments for their increased social service costs when illegal aliens receive legal status under the bill.
State and local officials, who say they already shoulder severe financial burdens because of illegal immigrants, want full reimbursement. Their intense lobbying, along with other objections to the program, has diminished support for the package painstakingly put together by Rep. Romano L. Mazzoli (D) of Kentucky and Sen. Alan K. Simpson (R) of Wyoming.
But the cost question remains: Do illegal aliens really take advantage of the US social-service system and will they, on amnesty, surge suddently onto the social-service rolls?
''The impression is that massive hordes of immigrants are coming here to take advantage of our social-service systems,'' says David Hayes-Bautista, associate professor at School of Public Health of the University of California at Berkeley. He and many opponents of Simpson-Mazzoli say erroneous assumptions are made about the burden undocumented aliens impose. If the aliens are undocumented , they reason, then presumably no well-documented statistics exist on any of their activities in this country. So, to the extent that the Simpson-Mazzoli immigration curbs are economically motivated, they do not have statistics to stand on, the critics say.
In his own 1983 health-care study of 357 Mexicans who had returned to Mexico after legally or illegally working in the United States, Mr. Hayes-Bautista found that only 5.8 percent received medical care at a county hospital. While 75 percent experienced some illness in the US, he found, only 42 percent sought medical treatment, and one-third paid for private medical care with insurance or their own money.
''Stereotypes to the contrary, our sample shows that Mexicans who come to the US temporarily - whether documented or undocumented - are largely not seeking public-sector services. To generate legislation based on that stereotype is to base it on a situation which does not seem to exist,'' he concludes.
Legal residency must be proved before applicants can receive welfare, food stamps, or Aid to Families with Dependent Children. Los Angeles County, for example, shows no record of undocumented aliens receiving any benefits from these programs.
It stands to reason though, says one informed observer, that if illegal aliens are here earning low wages they will need social services.
There is some specific evidence that illegal aliens use social welfare systems:
* More than 35,000 alien applicants for Aid to Families with Dependent Children (AFDC) are turned away annually in Los Angeles County. County officials see this as evidence that amnesty for illegal aliens would cost more money.
* Nearly 8 percent of all children on AFDC in Los Angeles County in 1981 had illegal-alien parents, says David Heer, a professor of sociology at the University of Southern California. He says the implication is that if the family financial condition qualifies legal residents for aid, then when amnesty is provided, whole families might qualify for more aid.
* David North of the TransCentury Foundation in Washington found that of a group of 192 aliens apprehended on the California border in 1975, 147 turned up later on state tax rolls. In later years 72 of them filed for unemployment insurance, and 51 received it.
Mark Tajima, a legislative analyst at the Los Angeles County Administrative Office, says that in 1983 two-thirds of all births in Los Angeles County hospitals were to illegal-alien mothers. While many may pay their own bills, the county estimates it pays $100 million in unreimbursed emergency and maternity costs. ''We'd like to see reform but we could not support a bill without reimbursements,'' he explains.
Joseph Nalven, whose San Diego company, Community Research Associates, has studied the fiscal impact of undocumented aliens, views the cost question in broader terms. ''How much cost can you attach to ethical principles?'' he asks.
Washington legislators may be able to differentiate between legal and illegal residents. But at the local level, he says, ''society has to ask how much restriction you put on the undocumented or all the poor.''