ALMOST NO ONE, it seems, is entirely pleased with the census. Central city congressmen, led by Rep. Mervyn Dymally (D-Calif.), want to force the Census Bureau to adjust its 1990 head count to compensate for the undercounting of people in central cities. Another, partially overlapping group of congressmen wants the Census to omit illegal aliens from its head count.

The arguments are growing hot because April 1, 1990, the date of the decennial census, is drawing nigh. The census results determine the apportionment of the 435 congressional districts among the states and provide the basis for drawing new district lines within each state. Moreover, billions of federal dollars are distributed according to formulas based on census results. No wonder the major lawsuit to require adjustments for the 1980 undercount was brought by New York city and state, which lost five congressional districts and billions in federal aid in the census. But there are -- or should be -- strong pressures on the other side. The census has a 200-year history of statistical integrity and political impartiality. Congress should be awfully reluctant to put that record in peril.

Both those who want to adjust the results for the undercount and those who want to omit illegal aliens have plausible arguments. But neither has a case that stands up on reflection. Statisticians agree that there is an undercount, but they inevitably disagree on precisely how to go about compensating for it -- a disagreement that invites politicians in to decide the matter according to their own best political interests. Those who don't want to count illegal aliens argue that neither political representation nor federal funds should go to those who are not citizens and are not legal residents. They include many congressmen from states such as Pennsylvania which have slow-growing populations and few aliens and the Federation for American Immigration Reform, which has brought a lawsuit to stop the bureau from counting aliens.

The trouble with their proposal is that it would have the Census Bureau get into an area where it does not belong: deciding who is and who is not an illegal alien. That task is difficult enough for the agencies charged with it. The Census Bureau is charged with counting, and if it is told to ask questions to determine who is behaving illegally, those people are going to avoid the census-takers. That would tend to increase the undercount, rather than reduce it as the bureau is trying to do.