One of the most profound effects of the Vietnam war on American attitudes has been a severe erosion of trust in government.

In 1964, when the United States entered the war, the University of Michigan Survey Research Center found that 76 percent of the people in its poll expressed a high degree of trust in the government "all or most of the time." That degree of trust, according to a recent poll by The Washington Post and ABC News, is now acknowledged by only 37 percent of the people.

This decline in faith was charted during the 1960s and 1970s in the Michigan studies. It dropped to 37 percent in 1974 at the height of the Watergate crisis and reached its nadir in 1980 when only one in four Americans trusted their government all or most of the time.

The gain in trust back to 37 percent, as recorded in the Post-ABC poll last month, has been largely in the upper-income groups, among Republicans and younger Americans -- 18 to 20 years old.

Popular mistrust of the government is reflected in the Post-ABC poll finding that Americans today by 2 to 1 believe that Presidents Lyndon B. Johnson and Richard M. Nixon deliberately misled the country about the war. And by a ratio of 55 to 41 they now say that American intervention in Vietnam was not worthwhile and by a 2 to 1 margin say the United States should not have become involved.

Some of the general public's attitudes toward the war are at variance with the opinions of Vietnam veterans who were surveyed in a separate Post-ABC poll. A majority of the veterans say they think the war was worthwhile. Unlike the general public, they overwhelmingly say the war could have been won and that more force should have been used.

The first of the Post-ABC News surveys dealing with Vietnam was conducted by telephone Feb. 22-26, with 1,506 people interviewed, selected at random in the continental United States. A second poll of the general public, in which some questions were repeated and others added, was taken March 21-25. It also entailed 1,506 interviews. From March 15 to 24, Post-ABC News interviewers did a random survey of 811 veterans who had served in Vietnam or Southeast Asia during the war.

If anything good came out of the war, as far as public opinion expressed in these polls goes, it is a belief on the part of many that the Vietnam experience has made the United States less likely to get involved in other major conflicts. About half the people surveyed in March said that was one result of the war -- and, by more than 2 to1, they said the nation was better off because of it.

The February survey asked whether Johnson was "almost always truthful in what he told the American people about the Vietnam war" or whether "he often tried to mislead the people about the war." The same same question was then asked about Nixon.

Fifty-seven percent said Johnson often misled the public; 23 percent said he was almost always truthful. Sixty-one percent said Nixon often misled the public; 27 percent said he was almost always truthful.

People did not always think the war was such a mistake. A Gallup Poll in early 1965 showed 64 percent saying the United States should "continue its present efforts," only 18 percent saying "pull our forces out" of Vietnam. By 1967, Gallup surveys showed growing doubts, but as late as February 1968, 61 percent described themselves as "hawks" who wanted to step up the military effort, and 23 percent as "doves."

That survey was taken at the opening of the North Vietnamese Tet offensive, a turning point. Soon after, sentiment had changed so sharply that Johnson announced he would not seek reelection. By November 1969, a Gallup Poll showed doves outnumbering hawks, 55 to 31 percent.

One of the more striking findings in the Post-ABC News surveys has to do with the present division in public thinking over whether the United States could have won the war under any circumstances. For years during the conflict, national debate raged over whether to get out of Vietnam or whether to stay and use more fire power. Poll evidence aside, few seemed to doubt that the United States could have won had it gone all-out.

Among the Vietnam veterans interviewed by the Post and ABC News, many voiced resentment at what they called the hindering of the military in a "politicians' war." Sixty-seven percent of the veterans still said the U.S. could have prevented the communist takeover.

Most citizens, however, appear to feel differently. Forty-eight percent in the March survey said the United States could not have prevented the communist takeover of South Vietnam "no matter how much military force we used," compared with 41 percent who felt the North Vietnamese and Viet Cong could have been stopped.

In addition, by 57 to 41 percent, those in the March survey of the general public said they had no clear idea "of what the Vietnam war was all about, that is, what we were fighting for." Among the veterans, however, 61 percent said they had a clear idea and 37 percent did not.

Among the most controversial aspects of the war was the coverage provided by the news media. Vietnam was the first conflict in which TV cameras were present for much of the action, and leading government officials frequently charged that news coverage undermined the war effort.

By a wide margin, however, the public is supportive, not critical, of news coverage of the war. Asked whether "the country was better off or worse off for having that war reported fully in the press and on TV," 60 percent in the February national survey said "better off" and 31 percent "worse off." In the March survey, the figures were 61 percent, better off; 32 percent, worse off.

Only seven of every 100 interviewed said they had taken part "in some kind of antiwar activity." Many, 41 percent, said they were critics of the antiwar movement at the time, while 30 percent said they were supporters and the rest neutral.

Over the years, many citizens who were once critical of the antiwar movement have had a change of mind. Two of every three of those interviewed said they think that "Americans who protested the Vietnam war were justified in their actions." Among those who said they were critics of the antiwar movement, 51 percent now say the protesters' actions were justified.

The public is evenly divided, however, over one of the more extreme forms of protest: refusal to enter the military service when drafted. Forty-seven percent said that young men who avoided the draft because they opposed the war were justified in their actions; the same percentage disagreed.

In the Post-ABC News polls of the general public, about six in 10 said the way the war was handled made them distrust the government in Washington at the time. Among those interviewed in February, 63 percent said they were at least as distrustful today; in the March poll, 72 percent said that.

About a third, 36 percent in one poll and 27 in the other, said their trust in government had increased since the Vietnam era. But even among them, suspicion is not uncommon: Asked how often they trust the government to do the right thing, four in 10 said only sometimes or never.

Among Vietnam veterans, 57 percent said they became distrustful of the government because of its handling of the war, and 73 percent said they were at least as distrustful now. Cynicism was even higher for veterans who saw heavy combat.

Many of the people interviewed by the Post and ABC News admit to having been relatively uninvolved with the war. Almost half, 44 percent, in the March general public survey said they did not often discuss the war with family or friends while it was taking place.

Twenty-two percent said the war affected their daily lives "a great deal" at the time; 24 percent said it affected them "a fair amount." But 36 percent said it affected them "hardly at all," and 17 percent said their lives were "not at all affected."

The least affected, naturally enough, were the youngest surveyed, those between 18 and 30 years old, who were children or youths while the war was fought. Among the young, 60 percent do not remember the war's being discussed much in their homes.

They are slightly more likely than older Americans to think the Vietnam war was a worthwhile cause, and more of them -- 48 percent in the February survey and 43 percent in March -- say they trust the government most or all of the time.

But only 57 percent of them knew which side the United States supported during the war. Many older people did not know that, either. Only 67 percent of all people surveyed could answer correctly that we supported South Vietnam, not the north.

If the war was one to which many did not pay attention at the time, it also seems to be one that many Americans have quickly forgotten.