JAMES SCHEUER, chairman of the House population committee, staggered by the massive flow of illegal immigration from and through Mexico, now calls for a "firm hard sealing" of the border. We can't agree. The problem is staggering: Pushed by poverty, pulled by opportunity, uncounted millions move annually across the world's only open border between a developing nation and a developed nation. Yet unilaterally sealing the 1.950-mile border would provoke a crisis with Mexico, a rising power with new cards-including oil-of its own, and with millions of "Mexamericans," too. It would require police measures of a sort hitherto unaccepted by Americans. It would change the nature and image of the United States as an open society.

All right, you may say: no "firm, hard sealing. "The alternative favored unanimalously by the population committee, in the immigration report on which its chairman improvised separately, is "rigorous border security. "We're not sure what that means. But such a condition, if enforced, would certainly to one thing. It would require the United States for the first time actually to decide which immigrants it wants and which it does not.

There is not national answer ready for the question. The relative suddenness of the swell and the mesmerizing image of the open border have left the country unprepared to cope with it. Only in the last few years have Americans begun to grasp its dimensions. Just in the last year has awareness spread that the issue cannot be treated apart from a host of others-energy, trade, Mexican development, the life of the border-straddling "Mexamerican" community-lying between Mexico and the United States.

One sign of this awareness is that the National Security Coucil, galvanized by hopes of nailing down future supplies from Mexico's newly and highly touted oil reserves, is preparing broad new options for overall American policy toward Mexico. The key question is whether Mexico should be promoted from regional ranking to something like world-class status in official American eyes. With 65 million people, an industrial base, those oil reserves and a strategic proximity, the real question is: Why not?

A second sign of budding American awareness is the population committee's report. In its readiness to cope with the immigration issue an to link a solution to difficult economic and political decisions north as well as south of the border, it is probably out in front of the American consensus. Out front is, of course, where reports like this ought to be.

This one recommends further diligent pursuit of the numbers that will allow intelligent choices to be made about its still dimly understood subject. It suggests establishing the border controls that alone will rescue policy from default. It calls for raising the level of legal Mexican immigration (now at 20,000) and exploring "guest workers. Correctly noting that the American choice is to accept either Mexico's farm and manufactured goods or its "surplus" people, it recommends that Mexico be accorded preferential treatment in American markets and "massive assistance" to absorb jobseekers at home. Unquestionably, these are the lines that future policy must explore.

One more word: The American government is poorly organized to cope with a problem like this one with so many tentacles. In the Congress, the House select population committee has been prominent among those trying to get a grisp. Its mandate, soon to expire, should be extended.