The chairman of the House subcommittee on the census will launch a campaign today to force the Census Bureau to adjust its 1990 figures to make up for an expected, severe undercount of blacks, Hispanics, American Indians and other minority groups.

Rep. Mervyn M. Dymally (D-Calif.) announced he will introduce legislation to require the Census Bureau to calculate how many people it misses on April 1, 1990, then add them into the official results reported to the president and Congress Dec. 31, 1990.

The adjusted figures under Dymally's bill would be used for all legal purposes and statistical summaries. Aides to Dymally, who is also chairman of the Congressional Black Caucus, said he has 42 cosponsors.

The Census Bureau and its parent, the Commerce Department, had no comment on the controversial proposal.

The stakes are large: the number of seats allocated to each state in the House for the decade of the 1990s will be based on the official count, and the boundaries of congressional districts and billions of dollars in federal grants also are decided on the basis of the census.

States with a heavy concentration of minority groups say the Census Bureau misses far more blacks, Hispanics and other minorities than whites. According to Dymally aide TerriAnn Lowenthal, the bureau estimated that it undercounted the national population by 1.4 percent in 1980, but that it undercounted blacks by 5.9 percent and Hispanics by almost as much.

As a result, areas with large black and Hispanic populations argue that they lost out on House seats and federal grant allocations.

New York state and city, in a 1980 lawsuit that is still unsettled, argued that the undercount of minorities cost the state a House seat and cost the city $26 million to $52 million a year in federal grant money. Other jurisdictions with large minorities have filed similar suits, such as Philadelphia, Detroit, New Orleans, Atlanta and the state of New Mexico.

An adjustment is expected to encounter hard going, both on political and technical grounds. While states with large minority populations would benefit, those with few would lose, such as some of the plains and mountain states.

Moreover, some Census Bureau officials are uncertain they can make an adjustment accurate at the block and neighborhood level, particularly if they must do so by Dec. 31, 1990. Others fear the census will lose credibility if the public gets the notion the numbers are being fiddled with.

However, Stephen Fienberg, dean of the College of Humanities and Social Sciences at Carnegie-Mellon Institute and chairman of the committee on national statistics at the National Academy of Sciences, said an adjustment can be made. The National Academy and an American Statistical Association panel have concluded that "it would be possible to produce quality data" down to blocks and neighborhoods.