THE RECENTLY published Census report on poverty in 1984 includes a glimpse of the future. The fastest growing group in the poverty population, as in the population generally, consists of Hispanics. Blacks remain the dominant minority among the poor. They are 12 percent of the population, but made up 28 percent of the poor last year. For Hispanics the figures are smaller -- about 7 and 14 percent. But the Hispanic percentages are rapidly rising. The Hispanic poor are concentrated in three large states: California, Texas and New York. To that extent they may be hidden from large parts of the general population. But the Census data, together with a new report on Hispanic children in poverty from the Congressional Research Service, can leave few illusions.

The overall poverty rate was 14.4 percent last year, for blacks 33.8 percent, for Hispanics 28.4. Increasingly in this country the poor are young; children now constitute more than a third of the poor. The poverty rate for black children was 46.2 percent for the year. For Hispanic children it was 38.7 percent. Among children of Puerto Rican descent, who make up a seventh of Hispanic children, the rate was 52.2 percent. How to walk back to mainstream America from that is not clear.

There are, however, fundamental ways in which the Hispanic and black poor differ. Much depends on how these are interpreted. Statistically, households headed by women are much more likely to be officially poor than households with adult males present; they are weaker economic units. The percentage of blacks in such units is higher than the percentage of Hispanics. Family structure is a more important part of black than of Hispanic poverty.

The CRS researchers found by contrast that "economic factors" may count for more in explaining Hispanic poverty. Hispanics tend to be recent arrivals. A third were not born here; another third's parents were not born here. Many have little education and have trouble with English. Their families tend to be large, and they tend to have them while young. The CRS group found that, perhaps reflecting all these factors, the poverty rate in Hispanic families where the father worked full time was three times that for comparable black families (though this was also true because the wives in the Hispanic families tended to work less).

The question these differences pose is what will be the pattern for Hispanics, the experience of blacks or that of other waves of immigrants. To what extent will Hispanic immigrants be able to work their way up the familiar ladder? To what extent are they likely to be kept on the bottom rungs? Some of the answer lies in the pending immigration bills; basic values are involved. There is no more serious issue facing the society, and the demographers cannot help.