A majority of Americans say they are watching less television than they used to, a nationwide Washington Post poll has found.

A marked decline in viewing was reported among more affluent and better educated people, among those who say the quality of TV programs is deteriorating, and among those who report the sharpest dislike for TV commercials. But a lesser decline in viewing also was reported among the bulk of the population -- among blacks and whites, women and men, young and old.

In all, 53 percent of those interviewed said they are watching less TV than they did five years ago, compared to 32 percent who say they are watching more now. If those figures are correct, or even close to being correct, they represent a striking reversal in the nation's entertainment and leisure habits. From the inception of TV until the mid-1970s, viewership had been reported as increasing year after year.

It is clear, however, that Americans' love affair with TV remains a marvel unlike any in the history of communications or entertainment. According to the poll, the average person 18 years old or older watches three hours of televison on week-days and three hours, 25 minutes on Saturdays and Sundays.

Only 1 percent of those polled said they had no working television sets in their homes; more than half said they had at least two working TV sets. One indicator of how much TV has become a part of people's lives lies in that last figure: in 1970 a survey reported that only 31 percent of the nation's households had two or more television sets.

Two-thirds of the 1,693 people interviewed by The Post said they watch at least some television every day, and 95 percent reported watching at least two days a week. Only 2 percent of those polled said they never watch TV at all.

Executives at ABC, CBS and NBC, apprised of these findings, said that TV viewing may have leveled off after having reached a "saturation point" several years ago. But they questioned whether people are watching less TV than before.

Jay Eliasberg, a vice president for research at CBS, said in an interview, "I know people are not watching less television, it's that simple." Saying that Nielsen reports and other industry-sponsored gauges of viewing supported his position, Eliasberg maintained that "except for tiny squiggles, TV viewing has not changed in the last three years."

The amount of viewing is vital to the networks because their income is largely based on total number of viewers. The network that has the most viewers charges more for advertising than the other networks and still has its advertising time gobbled up first; individual programs on all three networks that are thought to have more viewers than others are able to charge higher advertising rates.

Eliasberg and other TV executives say people are not the best judges of their own television-viewing behavior: If there is a stigma attached to watching television, a natural response would be to deny watching very much. However, one of two earlier landmark polls of attitudes toward TV, conducted in 1970 by Robert Bower, also asked people whether they were watching more or less TV than they had in the past. That study, which was initiated and supported in large part by CBS, found approximately 40 percent of those interviewed saying they were watching more TV, and 31 percent saying they were watching less -- evidence that people are not reluctant to say they are spending more time in front of the picture box.

The Post poll is not intended to state with finality people's reasons for watching less TV. It may be that, for some, the treat has worn off and other leisure activities are taking precedence over TV.

However, the poll does suggest that at least part of the reported decline in viewing is tied to strong feelings on the part of many who feel the quality of TV entertainment has been deteriorating. Overall, 40 percent of those interviewed said TV entertainment is better now than it was five years ago, and 41 percent said it is worse -- an even split, with the rest saying it is about the same as it was or offering no opinion.

Among those who said TV entertainment is worse now, 62 percent said they watch less than they used to. Among those who said the quality is better, 46 percent said they are watching less.

Overall, 17 percent said TV entertainment is much better today than it was five years ago, and they, almost alone in the population, reported that they are watching more TV these days. Even among them, however, the margin of increased viewing is slight, with 47 percent saying they watch more but 40 percent saying they watch less.

When people complain about the quality of TV, they seem to be focusing on the bulk of the medium's vast offerings and not on the shows they consider their favorites. People continue to get great pleasure from their favorite shows. What seems to have happened over the years is that more and more people are finding fewer programs that they regard as favorites.

Asked to name their favorite programs -- those they watch regularly or whenever they get a chance -- 30 percent couldn't think of any current leading show at all. Fifty-three percent could not name more than two shows as favorites.

In 1960 and 1970, half or more of those interviewed said they found at least 55 percent of the programs they watched to be "extremely enjoyable." In The Post poll, only 28 percent of those interv iewed gave such high ratings.

Asked how often they were disappointed with their favorite programs, fewer than three people in 10 said they were frequently or occasionally disappointed. More than seven in 10 said they were rarely or never disappointed.

The disappointment level for the rest of TV's offerings, however, was sharply higher.More than six in 10 said they were frequently or occasion ally disappointed with shows other than their favorites; only sone in three said they were rarely or never disappointed with them.

These numbers taken together, seem to draw a picture of a public that knows what it likes, and is finding less of it on TV than it used to.