THE SENATE is about to begin consideration of the Simpson-Mazzoli bill, a comprehensive revision of our immigration laws. The proposal is the result of years of study, extensive hearings and wise compromise on some of the more controversial aspects of this problem. One of the most important provisions is designed to control illegal immigration by penalizing employers who hire undocumented workers. Such a sanction, sponsors of the bill believe, is the only way to control borders, since most illegal immigrants come here specifically to work.

The Select Commission on Immigration and Refugee Policies, created by Congress in 1979, estimated that between 4 million and 9 million undocumented workers are now in this country, but former labor secretary Ray Marshall, who was a member of that commission, admits that the estimate is a result of a compromise among a widely varying set of guesses as to the actual number. Officials simply don't know how many illegal aliens are here, but they do know that unemployment in this country is now at 9.8 percent, and that every job held by an illegal alien is one not available to an American citizen.

Many employers oppose any change in the present law-enforcement system since they prefer to pay very low wages to workers who cannot avail themselves of their rights. Mr. Marshall and many labor leaders believe that the only way to preserve these jobs and improve wages and working conditions in some industries is to eliminate that option for employers. He is right.

Another controversial question concerns refugees. Under the provisions of the Simpson-Mazzoli bill, 425,000 new immigrants would be allowed to enter the United States each year. This ceiling does not include refugees who, under existing law, may be admitted in any number on the authority of the president as long as he notifies Congress of his intentions. Such flexibility is needed to deal with emergency situations where quick action must be taken for humanitarian reasons.

It was assumed that under the provisions of the law, about 50,000 refugees a year would enter the country, but for a variety of reasons -- the continuing needs of Indochinese refugees, the Cuban boat lift -- that figure has been much higher in recent years. Sen. Walter Huddleston (D-Ky.) would apply the 425,000 ceiling to immigrants and refugees combined. He would continue the president's flexible power to meet emergency situations by allowing large numbers of refugees in during any given year, but would then subtract numbers over the ceiling from the following year's quotas.

Those supporting the Huddleston position believe that Americans are suffering from "compassion fatigue," that we are already doing more than our share to accept the homeless and the persecuted of the world and that, for purposes of our own long- range economic planning, we must have a firm and fixed number of new admissions to the country. To disagree with this position is not to accuse its proponents of mean-spiritedness.

The United States has accepted on a permanent basis large numbers of refugees in recent years, and it has not always been easy. And we should take care that the total numbers do not regularly substantially exceed the original expectations of Congress. Nevertheless, the Huddleston amendment should be rejected because it will inevitably curtail this country's ability to accept its share of the world's refugees. In stark political terms, if refugees have to compete for quota numbers with the brothers and sisters of American citizens, there is no doubt who will be admitted.

This nation was founded as a haven. That quality is part of our national character, and there will always be room here for those who come without connections and without special skills simply because they must come here to survive. The Huddleston amendment severly restricts that tradition when a prudent application of existing law ought to be enough. The Simpson-Mazzoli bill should be passed without the Huddleston limitation, and the House should concur.