A lawsuit to block the Census Bureau from counting illegal aliens in the 1990 census for the purpose of reapportioning House seats will be filed today in U.S. District Court in Pittsburgh by the states of Pennsylvania and Kansas, joined by 40 members of Congress and two public organizations.

The suit, announced at a news conference by Rep. Thomas J. Ridge (R-Pa.), is the first shot in an expected protracted series of legal battles over the 1990 census. Other lawsuits by different groups alleging a severe undercount of blacks and Hispanics are also expected.

"In my view," Ridge said, "the practice of including illegal aliens in the count that determines reapportionment of congressional districts is not only unfair, not only illogical and ill-conceived, but also unconstitutional."

The bureau yesterday had no comment on the pending lawsuit.

At the heart of the dispute is the question of how many House seats each state will receive for the 1990s. Every 10 years, the 435 seats are reallocated among the states on the basis of population. In 1980, about 2 million illegal aliens were estimated to have been included in the census count.

The Federation for American Immigration Reform (FAIR), which is joining in the suit along with the Coalition for Constitutional Reform, said that if 2 million or so illegal aliens are again included, "Pennsylvania, Kansas and West Virginia may lose congressional representation as a direct result of counting illegal aliens in the 1990 census. Other threatened states include Connecticut, Michigan, Ohio, Alabama, Missouri and North Carolina." California and Texas, on the other hand, may gain seats.

That is why Pennsylvania and Kansas, plus 25 members of Congress from those states and some from other threatened states, like West Virginia Democrats Harley O. Staggers Jr. and Robert E. Wise Jr. and Connecticut Democrat Barbara B. Kennelly, have joined in the suit.

The lawsuit contends that "including illegal aliens in the population base for apportionment is unconstitutional, because illegal aliens, who are not entitled to representation in Congress, are not to be counted as 'persons' for apportionment purposes under Article I Section 2 of the Constitution as amended by the 14th Amendment."

The constitutional argument against including illegal aliens in the count has been made in the past, for example, in a 1979 FAIR lawsuit, but that suit was dismissed. The Census Bureau has announced that it intends to count every person regularly residing in this country that it can find.

The lawsuit announced yesterday said the bureau need not exclude illegal aliens from census counts used for land planning purposes or distribution of federal grants, only from the count used for apportionment of seats in the House.

TerriAnn Lowenthal, staff director of the House subcommittee on the census, said Chairman Mervyn Dymally (D-Calif.) "believes that the Constitution does not permit the exclusion of undocumented residents from reapportionment counts."