With the specter of unemployment hovering over much of the nation and illegally employed aliens a controversial aspect of that problem, a report recently completed by the Rand Corp. offers a startling view of the United States in 20 years: It will be a country whose shrinking work force requires it to encourage immigration.

The report, a summary of a decade's worth of research at the California-based think tank, coincides with Senate deliberations on fixing immigration quotas. It predicts that the number of young workers in the country will decline dramatically because of the "baby bust" that began in the early 1960s.

The low U.S. birth rate could lead in 20 years to high wages, low unemployment and a burgeoning job market competing to hire immigrants, Rand demographers said.

The Senate is expected to vote today on a bipartisan bill that would set immigration quotas, impose jail penalties on employers who hire illegal aliens and grant some amnesty to illegal aliens already in this country.

"If we set immigration policies and quotas that are very restrictive, we may find ourselves in 10 or 15 years needing more workers and having to go through debate to relax those standards," Rand demographer Kevin McCarthy said. "One of the things immigration does is supply labor, cheap labor, to the market."

He said the congressional debate may be shortsighted, adding, "I think the conditions facing us right now, the debate, the hullabaloo and the climate that produced it will shift dramatically."

The 39-page report is a summary of internal research at the think tank and was not produced for a particular client. The research was based primarily on statistical data from the Census Bureau and current population statistics.

"Legally or illegally, we're in for larger streams" of immigrants, said Bill Butz, senior Rand economist. "The purpose of the report is just to warn that the pressure is rising."

Butz added that "immigration is one of three safety valves for the labor market." The other two groups that could supplement a shortfall in the labor force, he said, are the elderly and nonworking women. "We foresee a reversal in the trend to early retirement," he said.

When asked about the current levels of black unemployment he predicted that many black teen-agers, currently unemployed, would be absorbed gradually into the job market. "A major part of their problem is not demographics but the recession," he said.

As for skilled workers, "It may seem hard to believe for someone who is unemployed now, but over the long run of a decade the underlying demographics say they will find work."

One demographic condition that is not expected to change, according to the Rand report, is the current low birth rate. In 1800 the rate was about seven children per woman, but in 1980 the figure was two children per woman.

The report also raised the possibility that the armed forces would need immigrant recruits to fill its ranks, as the draft-age pool shrinks over the next decade. In that case, he said, the qualifying examinations might need to be changed to accommodate immigrants unfamiliar with the language and not educated enough to handle sophisticated weaponry.

"The same demographic and labor market conditions that induce foreigners to migrate here may also reduce the opposition to accepting them," the report said. "Even the traditional opposition of labor unions may abate."