Americans give the news media high marks for credibility and competence and think their "watchdog" function helps protect democratic liberties but believe that the media are influenced too much by powerful institutions such as government and big corporations, according to a survey commissioned by the Times Mirror Co.

By 70 to 30 percent, respondents in the exhaustive study have a positive overall opinion of newspapers, magazines and broadcasters -- about 90 percent give them a high grade for "believability" -- but this wide support is relatively shallow. Critics' negative feelings are considerably stronger and deeper than supporters' positive views.

About half the sample, 45 percent, said the media were biased in reporting. Forty-one percent described news organizations as liberal, 19 percent as conservative; 22 percent of the sample said reporting is liberally biased, while 9 percent said it has a conservative slant.

The most vociferous critics, who make up about 5 percent of the population, were found likely to be better educated and informed than supporters, more likely to have been in the news and more outspoken. They generally consider themselves members of the Establishment and are more likely to view the media as antagonists, the study concluded. They are also much more Republican than Democratic, and more conservative than liberal.

"This may help explain why members of the press perceive much less support in the public than actually exists," said Andrew Kohut, president of the Gallup Organization. "It cannot be comforting to the news media that those who know their product best and use it most are those who support it least. But there simply is no credibility crisis between the press and the American people."

The survey cited two major reasons for this:

*Americans like the news, and believe that competition among the news organizations results in a more complete and better report. The survey showed, for example, contrary to much criticism, that opinions of the media were 5 to 7 points more favorable during the TWA hostage crisis than in the two months after it.

*They also put a high value on the media's role of "watchdog" over government, politicians, big business and the military. But a majority -- 53 percent -- said the media, far from being too powerful and arrogant, are too easily manipulated by these institutions.

Gallup's interviews with a national sample of 2,104 indicated that most Americans have mixed feelings about the media. Pollsters, struck by what they called these "enigmas," doubled back and reinterviewed respondents in search of clarification. The pollsters also conducted panel discussions in New York, Chicago and Los Angeles.

The survey found that positive and negative attitudes alike cut across all demographic, ideological, economic, social and political lines. The attitudes generally were shared almost equally by men and women, liberals and conservatives, Republicans and Democrats.

The survey found that "We like the news and we like the people who bring us the news," Kohut said. "We believe and value what they tell us, even while we also believe they can be rude, biased, subject to outside influence and prone to other sins as well."

Americans are also more supportive of First Amendment freedom than many in the media believe, the study shows, but they define this freedom as their right to be informed. When asked, 61 percent said it meant the public has the right to hear all points of view while 23 percent defined it as the press's right to report what it chooses.

Two-thirds, 66 percent, said they would favor publication of the Pentagon Papers compared to 21 percent opposed. But when asked about a magazine article (in The Progressive) describing how to build a hydrogen bomb, 52 percent opposed publication on grounds that it would compromise national security; 35 percent favored publication. Two-thirds, 67 percent, said criticism by the media "keeps leaders from doing things that should not be done," compared to 17 percent who said it "keeps leaders from doing their jobs." A majority, 51 percent, said such criticism "keeps our nation militarily prepared," compared to 31 percent who said it "weakens the country's defense."

In the "believability" ratings, the three major broadcasting networks and The Wall Street Journal scored 86 or 87 percent, the news magazines and local television and newspapers a point or two lower. The network news anchormen scored about 90 percent believability. Walter Cronkite, although in retirement, led them all with 92 percent.