Americans oppose by almost 4 to 1 any U.S. involvement in attempts to overthrow the leftist government of Nicaragua, a new nationwide Washington Post-ABC News poll has found.

The percentage of opposition is as high as or higher than in the past, despite recent efforts by President Reagan to marshal support for pressure against the Sandinistas. In the survey, conducted Friday through Tuesday, 70 percent said they oppose U.S. involvement in efforts to topple the Nicaraguan government, 18 percent said they favor it, and 12 percent expressed no opinion.

On other issues:

* More than three out of every five persons interviewed judged that the average farmer is worse off financially than most Americans, and four of five said they think the government should extend farmers at least as much special aid as the administration is proposing.

* One of every three interviewed said an immediate family member will be affected if the administration succeeds in cutting back loans to middle-income college students. Among that third, one in three said the result would be abandonment of plans to attend college.

* Citizens are sharply divided over the designation of Edwin Meese III as attorney general, with 43 percent saying they disapprove, 38 percent saying they approve, and 19 percent expressing no view. Meese was confirmed by the Senate Saturday, 63 to 31, with all the "no" votes coming from Democrats.

As for views toward Nicaragua, U.S. involvement in attempts to overthrow its leftist government is opposed overwhelmingly in all regions of the country and by all segments of the population, including strong backers of Reagan. Republicans, for example, oppose intervention by 60 to 26 percent -- only somewhat less than Democrats' 76 to 12 percent.

Perhaps most striking is that this opposition is so high at a time when Reagan is going out of his way to suggest that the Sandinistas should be overthrown.

Last Thursday, the day before the Post-ABC News poll began, Reagan held a news conference during which he called the Sandinista regime brutal and cruel. Asked if he advocated its overthrow, he responded: "Not if the present government would turn around and say, 'All right' -- if they'd say, 'uncle.' "

The previous week, the president used his Saturday radio speech to denounce the Nicaraguan government and to praise the "contras" fighting it.

Fifty-nine percent of those interviewed said they believe that Reagan wants the United States to be involved in attempts to overthrow the Sandinistas.

But -- though the survey is not conclusive on the point -- it appears that opposition to U.S. involvement is at a higher level than in any of three other Post-ABC News surveys in the past year and a half.

In August 1983, 62 percent of those interviewed disapproved of U.S. involvement in overthrowing the Sandinistas. In November 1983, the figure was 48 percent, and in January 1984 it was 55 percent -- all substantially below the 70 percent opposition recorded in the latest survey.

One factor that could account for at least part of the difference is a slight change in the wording of the question asked in the poll. The new survey asked: "Should the United States be involved in trying to overthrow the government in Nicaragua, or not?" In the earlier surveys, before asking that, the questioner noted that "the United States, through the CIA, is supporting the rebels."

On the Meese nomination, the public, like the Senate, divided along party lines. Among Democrats, 58 percent disapproved and 26 percent approved; among independents, 41 percent disapproved and 35 percent approved, and among Republicans, 58 percent approved and 25 percent disapproved.

The survey also asked whether respondents had favorable or unfavorable impressions of three of Reagan's other chief aides: Secretary of State George P. Shultz, Secretary of Defense Caspar W. Weinberger and budget director David A. Stockman.

Shultz got the highest positive rating, with 49 percent saying they have a favorable impression of him, 14 percent an unfavorable impression, and the rest expressing no opinion.

The figures for Weinberger were 33 percent favorable and 21 percent unfavorable; for Stockman, 28 percent favorable and 28 percent unfavorable.