In offering a home-buying program, Gov. Michael Dukakis has focused on an issue that conceivably could be decisive in how certain groups vote.

Among them are voters in the 20- to 40-year age brackets, and young singles and couples who want to own but rent instead because they cannot afford the down payments.It is an all-American issue, along with flag and parenthood, intertwined with ideals Americans admire, including individualism, independence, freedom, security, wholesome family life and success.

But if it is an all-American goal, it is a faltering one. Home ownership isn't what it used to be. After climbing from 43.6 percent in 1940 to 65.8 percent in the third quarter of 1985, it now rests at 63.7 percent.

The full significance of those numbers isn't apparent, however, until the breakdown is analyzed. Then it is found that the most pronounced decline in home ownership is concentrated in the younger households.

In some respects, the figures suggest a growing split between the haves and the have-nots, the former being the older households who got into the market before the big price increases, and the younger households who are shut out.

Among the haves there is no group with a higher rate of home ownership - 80.7 percent - than those aged 60 to 64. But every five-year age group from 40 years to 75 years and older has an ownership rate in excess of 70 percent.

Now the contrast: Among those in the 25-29 age group, only 36.3 percent owned homes in the first quarter of 1988, according to the National Association of Home Builders.

Confronted with those numbers, some older homeowners contend that younger people should not expect to own houses, often adding that young folks today do not understand patience. "Let them save for it like I did," they say.

But the fact is that it was easier for a young couple to buy a house 15 years ago. Throughout the 1970s, the ownership rate in the 25-29 age group was regularly near or above 40 percent. Annual declines were unknown.

Since 1981 the ownership rate in that age group has declined every year but 1984, and that exception was only by a small fraction of a percent.

In the age 30-34 category there has also been a sharp decline, from 62.6 percent in 1978 to 52.8 percent in 1988's first quarter.

Cold figures describe a hot issue.

For one thing, it is households in these age categories that are most often used to illustrate the American dream of happy homeownership. The sense of denial, therefore, is enhanced by daily repetition of what might have been.