Americans hold sharply contrasting views on the labor union movement: they are highly critical of union leaders and concerned that unions have too much power in public life, but they admire the accomplishments of unions and want them to thrive.

Sentiment toward union bosses is so negative that a candidate for high office could conceivably win election by getting union leaders to endorse his opponent. Twice as many people say they would vote against the union leaders' candidate as for him.

At the same time, more than two out of every three people approve of unions in general and a strong majority say workers are "better off belonging to a union than not." The idea that unions may have been needed at one time, but not any longer, is rejected by 60 percent of the public.

These are among the chief conclusions of a new Washington Post-ABC News poll on public attitudes toward labor unions and other matters. They come at a period in American life when many observers feel the public has rejected the union movement as a force of progress, often citing public opinion polls to make the point.

In the conventional wisdom, unions are seen as often tied to mobsters and union workers as so protected by their contracts that they don't have to work hard. Labor demands are sometimes portrayed as being more responsible for inflation than the practices of government or business.

The new poll, through a series of more than 30 questions on these issues, suggests that these perceptions are shared by large numbers of people. Fifty-one percent say that union members don't work as hard as nonunion people, for example, and only 14 percent say they work harder. Nevertheless, the public finds more to praise about unions than to criticize.

One question, asked only of people who said they belong to a labor union--14 percent of the sample--was this: "Overall, how would you rate the accomplishments of that union: excellent, good, not so good, or poor?" Fifteen percent say excellent, 64 percent good, 15 percent not so good, and 6 percent say poor.

For people who had no association with a union, this question was asked: "If you were working on a job where you could join a labor union, do you think you would join, or not?" The answer: 51 percent would, 40 percent wouldn't, and 9 percent are undecided.

The poll shows that unions are widely credited for improving pay and working conditions today, as well as in the past. Two-thirds agree with the statement that "labor unions ensure fair treatment for workers," four in five agree that unions improve wages, working conditions and job security. By 70 to 24 percent, the public disagrees that wages and working conditions would have reached today's level without labor unions.

Nevertheless, criticism of union power and bosses is widespread. A majority agrees with the statement that "labor unions control, dominate or run business." Three-quarters see unions as organizations that bring power or money to union leaders.

Asked whether ties between unions and gangsters are exaggerated or whether most national unions do have ties to gangsters and hoodlums, a majority says such reports are exaggerated. But fully one-third feel the ties exist.

Forty-five percent say that unions "have a great deal of influence in how the country is run," and a majority says unions should be less influential than they are. About four in 10 people want unions to have the same power as business in the United States--which, in the public thought, would represent a greater decrease of influence for business than for labor.

Almost six in 10 of those interviewed say they personally are more sympathetic to labor than to business; just over three in 10 say they sympathize more with business. In near unanimity, citizens see President Reagan as pro-business, with 74 percent saying he is more sympathetic to business than labor, and 10 percent saying he is more sympathetic to labor.

Along those lines, there is somewhat less approval of Reagan's handling of the nationwide air controllers' strike than existed last August, when the president was widely applauded for firing controllers who would not return to their jobs. Forty-two percent in the new Post-ABC News poll say Reagan has been too harsh with the controllers; 50 percent say his treatment of them has been "about right," and 5 percent say he has not been harsh enough.

On another matter, three of every four people, including the same proportion of union members, say they approve of unions accepting cuts in wages and benefits if necessary to protect the jobs of union members. A majority of people in families where there is a union member expect companies to ask for such concessions the next time their union engages in contract talks.

Apprised of the findings of the Post-ABC News poll, Murray Seeger, a spokesman for the AFL-CIO, the nation's main union organization, said negative appraisals of unions in the poll "are not surprising at all," and cited the news media as partly responsible for such views. He said the public's view of Reagan's policies regarding labor "is an improvement for us, in a political sense." By Barry Sussman, Washington Post Staff Writer; Staff writer Warren Brown and polling assistant Kenneth E. John contributed to this article.