THE REAGAN administration has performed the formidable bureaucratic feat of assembling a full package of immigration reforms. This puts it in a position to join Congress, which for some time has been seized of the issue -- principally the issue of illegal immigration -- in thrashing out a new national policy. For this people should be grateful, for immigration has been allowed to get scandalously out of control. Unfortunately, however, the administration's package is not impressive. It is "realistic" -- its own congratulatory designation -- to the point of likely ineffectiveness. Its proposals to deal both with old and with prospective illegals are inadequate.

Old illegals: there are an estimated three to six million, most of them productive and rooted. Some kind of amnesty is everyone's answer. The administration's kind might serve a certain deterrent purpose by advertising that American citizenship is a tough case to crack. But it likely would not serve the more important intended purpose of legalizing the existing crop. They would have to announce their presence, wait for 10 years, agree not to bring in relatives, pay income and Social Security taxes but go without federal social benefits, and learn English. How many illegals would find these terms an improvement on their current status? Would not many (most?) stay underground, thus continuing to advertise the allure of illegal immigration?

Prospective illegals: the administration would in effect legalize many of the Mexicans who now come illegally. It would raise Mexico's annual quota from 20,000 toward 70,000; this makes sense, not least diplomatic sense. It would begin a 100,000-person test of "guest workers"; this flies in the face of the bitter experience of worker exploitation that led the United States to end its 22-year bracero program in the 1960s.

Otherwise, the administration's measures to bar illegals are flimsy. The dramatic but slight flow from Cuba and Haiti would stand a good chance of being intercepted at sea. But the routine and huge flow by land would barely be touched by the minimal improvements contemplated in border patroling. Most important, the administration has rejected congressional appeals for the basic enforcement mechanism of a secure non-discriminating worker identifier and a tough law penalizing employers for hiring illegals. Instead, it suggests use of the same conventional, easily forged documents and the same weak civil penalties that, on the state level, are proven failures.

It is, finally, on the basis of the "social contract" that the administration's new policy is inadequate. Most of the immigrants of concern come in at the bottom, adding something valuable and desirable to American society as a whole but often providing tough competition in jobs, housing and the like to Americans at the bottom. These same low-income citizens happen to be the Americans most hurt by this administration's budget cuts and least helped by its tax cuts. This is unfair.

Whether a specific numerical ceiling should be set on immigration is a reasonable question. The administration slides by it, and so, at least for now, do we. But certainly the prime political consideration ought to be the impact of immigration policies on those Americans who will be most affected by the flow. Congress in its deliberations has come notably closer to this guideline than has the administration. A joint search for the right national policy should now begin.