Illegal aliens: rising US jobless rate revives issue. Key question: do Mexican laborers really deprive Americans of work?

Viewed from a peaceful hillside, the bustling city below is wrapped in a thick, midday haze - exhaust pouring from endless miles of cars, trucks, buses, mixing with the clouds emitting from industries in what has become one of the world's largest and most dynamic metropolitan centers.

But Mexico City has also become - once again - the focal point in an escalating conflict between the United States and Mexico over the issue of illegal migration into the US.

Although most US public concern and attention has focused on the 1980 deluge of Cubans into the US and the periodic arrival of boatloads of Haitians, most experts say they believe at least half the illegal aliens now in the US are Mexicans.

And with US unemployment now at 8.9 percent, representing more than 9 million Americans out of work, the question of regaining control of US borders again has moved into a prominent position on the US political agenda.

A showdown between the two nations on this controversial issue appears to be in the making. Neither side seems anxious for a confrontation, but with economic times tough in the US some key politicians feel pressed to take action.

''We're going to do something,'' Sen. Alan K. Simpson (R) of Wyoming, chairman of the Senate subcommittee on immigration, said in a Monitor interview here in Mexico City. ''I don't know if it will be right or wrong.''

Then he added: ''We hope we can defuse this emotional issue.''

The key sticking point: jobs.

Do illegal aliens take jobs away from Americans or do they fill jobs Americans do not want? Are the illegal or ''undocumented'' aliens a boost or a burden to the US economy?

There is strong disagreement on these questions.

US labor leaders contend there would be fewer Americans out of work today if there were fewer undocumented aliens in the country.

But some US scholars contend just the opposite is true. They argue that if businesses, farms, and ranches dependent on undocumented aliens were to lose that help, many of them would collapse. And that would mean loss of jobs for the legal residents who work in them, these analysts assert.

Further, it is claimed, many jobs are of such low social status that US citizens and legal residents would not take them. But, counters an AFL-CIO spokesman and a US Department of Labor analyst, without the undocumented workers , those businesses that survived would be forced to raise wages - thus attracting legal residents to the work.

There is an even more fundamental area of disagreement. It concerns the number of undocumented aliens in the US today.

During the mid-1970s the US Immigration and Naturalization Service (INS) frequently cited a figure of 2 million to 12 million undocumented aliens in the US. The INS later revised its top figure down to 8 million. Today these figures are considered by INS and Census Bureau statisticians and most US scholars on the topic to have been gathered by unreliable methods.

The US government figures now read 3 million to 6 million illegals. These estimates are not based on original research, says an aide to Senator Simpson, but on an analysis of previous studies.

The Mexican government's extensive study estimates the number of Mexicans in the US illegally at any one time ranges from at least 500,000 to 1.22 million, with the high point coming in the summer and the low in the winter.

If Mexicans do make up half of the illegal aliens in the US - a number that is in itself only a rough estimate - that would mean about 1 million to 2.4 million undocumented aliens.

For the moment, as Senator Simpson found on his visit here, Mexican politicians from President Lopez Portillo on down are making an orchestrated effort not to get involved in the US debate over the undocumented workers. With US unemployment figures up again, Mexico's leaders view this as a poor time to enter such a debate.

And besides, says Mexican Department of Labor Research coordinator Carlos H. Zazueta, ''I think it (immigration) is another difficult issue (between the US and Mexico, but) it is not the most important.''

Trade, oil, and Central American unrest should, according to experts on both sides of the border, have their own prominent places on the agendas of both countries.