The Census Bureau, which missed 5.3 million blacks, Hispanics and other Americans in its 1970 headcount, is embarked on a major efforts to find them in 1980.

"It's the No. 1 priority for 1980," said bureau Associate Director Daniel B. Levine after a sometimes acrimonious give-and-take in two days of meetings last month with a Spanishheritage advisory committee at the bureau's Suitland, Md., headquarters.

The 1970 undercount represented about 2.5 million: 1.9 per cent for whites but 7.7 per cent for blacks. Breakdowns don't exist for other minority groups, but census officials say the undercount is probably high among them, too.

More is at stake then statistical accuracy. Allocation formulas for many faderal aid programs are pegged to population breakdowns, and "lost" people mean lost money: millions of dollars a year to central cities and other areas heavily populated with disadvantaged minority groups.

To improve the count in 1980, the bureau is mounting what Levine calls an unprecedented effort to solicit cooperation of minority group members, who have historically been undercounted for a variety of reasons, including cultural and language differences and fear of government.

The bureau is preparing to recruit more census enumerators from local neighborhoods and sharpen questions aimed at eliciting accurate information on ethnic heritage. It has created a Minority Statistics Program, with community workers operating out of 12 regional offices to increase public awareness of the census and dispel hostility to it.

It has set up advisory committees from the black, Spanish-origin and Asian-Pacific Islands populations, and its top officials meet regularly with them - an exercise in "creative selfflagellation," as one official describes the sessions.

The session with the Hispanic group, for instance, resulted in a protest to Secretary of Commerce Juanita M. Kreps, who oversees the Census Bureau, because the bureau wouldn't accept its wording of a new question about ethnic heritage for Spanish-origin groups.

The meeting also produced a complaint the the bureau cannot expect Mexican aliens who are living illegally in the country to cooperate with census takers if the Immigration and Naturalization Service swoops down on them shortly afterr they talk to the enumerators. The happened during a dry-run test of the census in Texas last year, a committee member alleged.

Complaints about undercounting are as old as the Republic.

According to Henry H. Smith, the bureau's information officer, the first recorded complainant was George Washington, who faulted the first census in 1790 for failing to show a population of more than 4 million, which he had anticipated in hopes of proving the new nation's strength to a suspicious world.

But heightened sensitivity to racial and ethnic identities, coupled with the increasing importance of population breakdowns in dispensing federal funds through revenue-sharing and other programs, made the matter a dominant concern during the late 1960s and 1970s.

The illegal allien problem is one that makes counting the Spanish-origin population as difficult, if not more so, as counting other minority groups.

The Immigration Service estimates that 6 million to 8 million aliens are living illegally in the country, about three-fourths of them (4.5 million to 6 million) coming from Mexico alone.

The Census Bureau calculated the Spanish-origin population, including Puerto Ricans, Cubans and South Americans as well as Mexicans, at 9 million to 9.5 million in 1970. As of March, 1976, the Hispanic population was about 11 million, including 6.6 million of Mexican heritage, according to a sample survey byt the bureau.

If the Immigration Service's estimate is correct, the census is clearly missing a lot of people of Mexican origin, legal residents and otherwise.

The Census Bureau readily coucedes it has no idea how many people of Spanish origin it is missing, although it can make an estimate of the population as a whole and the major black-white breakdowns by using birth and death statistics, immigration figures and other data from separate sources.

It has figured that it failed to count 7.7 per cent of the black population because racial designations on birth and death certificates and other official papers give it a standard of comparison. However, such figures are not available for ethnic groups such as Hispanics.

Some bureau officials concede that, in percentage terms, the Spanish-orgin undecount may exceed the black undercount, although they are careful to make no estimates.

"Sheer common sense tells us we're missing a very large number," said one official.