The next president, if he is to avoid a term of "drift and muddle," must confront several bleak social trends that mar the landscape of an otherwise "comparatively well off" nation, a study by the Urban Institute warned today.

The trends documented in the study include: slow economic growth despite recent improvements; stagnant family incomes that threaten the capacity of young families to reach for the American dream; persistent poverty and the rise of an intractable "underclass," and burgeoning health and pension costs for an aging population.

"Fears of becoming a second-rate power are largely exaggerated," wrote Isabel Sawhill, editor of the study. "The concern seems to be that eventually Japan (or some other industrialized country) will replace us as the major economic power. This seems unlikely, given the size of our population and the tendency of growth rates to converge over time."

The study, "Challenge to Leadership," said the new president must give full attention to some "unavoidable domestic issues of the next decade."

The president's attention, however, will likely be diverted by the massive budget deficits left by President Reagan's administration. Rudolph Penner, former director of the Congressional Budget Office, and economist Joseph Minarik suggest ways to cut federal deficits $150 billion a year by 1992 -- half by cutting federal spending and half by selected tax increases.

Sawhill said that "if history is any guide, a recession during the next four years is almost inevitable," ruling out any "quick fix {or} magic bullet" to solve international or domestic problems.

The study contains no formal proposals by the Urban Institute, a nonprofit research organization, but many suggestions and recommendations from Sawhill and other contributors.

One of the most startling -- because it would be a major break with present practices and because it comes from a group of moderate-to-liberal economists -- is that the elderly be asked to pay for a sizable share of future Medicare and Social Security program costs and improvements.

The study said this could be done by making a larger portion of the monthly Social Security benefit received by retirees subject to federal income tax, by imposing higher Medicare premiums pegged to the individual's income, and possibly by slowing down increases in the formulas used to calculate each person's basic initial Social Security benefit.

John Palmer, a Carter administration official who wrote the section on Social Security and Medicare, said that restraining basic Social Security benefit growth would be a last resort and that direct means-testing of the two programs, converting them into welfare, would be a bad idea.

Palmer said elderly income is now "more-or-less on a par with that of the non-aged," and therefore, taxing a larger share of Social Security benefits and relating Medicare premiums to an elderly person's income would be "in line with the growing capability among some of the aged to shoulder more of the burden for their retirement and health care."

In its statistical portraits, the study outlines some major challenges:Economic growth slowed substantially from 1973 to 1986. For a variety of reasons -- which may include the energy crisis, the baby boom that produced a drop in the skilled proportion of the labor force and retarded growth of the capital-to-labor ratio, and a reduction in government research and development -- "the economy grew at a 3.7 percent annual rate in the period 1948 to 1973 and only at a 2.4 percent rate from 1973 to 1986." Many of the other problems are caused in part by this lagging growth rate. The income of the typical family measured in 1986 dollars, according to Minarik, jumped from $15,492 in 1954 to $27,770 in 1973, or 79.3 percent. But by 1986 it had risen to only $29,458, or 6.1 percent higher. Thus, median family income, he said, grew 3.1 percent a year in 1954-73, but only 0.5 percent a year in 1973-86. Even after factoring in the growth of fringe benefits, income grew slowly during the 1973-86 period while taxes and typical elements of the American dream, such as the carrying costs of a house, grew rapidly. There was a marked increase in income inequality. Minarik said that in 1954, the lowest one-fifth of families received 4.5 percent of total money income, the highest 41.8 percent. Over the next 15 years the lowest group gained and by 1969, its share was 5.6 percent and the top group's 40.6 percent. But the trend reversed and the lowest quintile sagged to 4.6 percent in 1986 while the highest rose to 43.7 percent -- the greatest gap of the post-World War II era.

Minarik said that despite much speculation, there is "no obvious cause" and "no clear villain" to explain the slowdown in the growth of family income, earnings and the greater disparities of income distribution. "In particular, the alleged 'deindustrialization' of the American economy has had virtually no measurable effect," Minarik said. Despite many years of antipoverty efforts, Sawhill finds the proportion of Americans below the official poverty line in 1986 was still 13.6 percent (32 million people) -- about the same as the end of the 1960s, higher than any year in the 1970s, and higher than most other industrialized nations for which data are available.

Sawhill said that antipoverty and income-support programs have been effective, especially among the elderly. But she said their effects have been offset by poor performance of the economy, high unemployment rates, the increasing maldistribution of income and an increasing proportion of female-headed households, whose incomes are usually low.

Although efforts to aid the poor should continue, she argues that the highest priority should be given to combating the growth of the "underclass" -- "a small but rapidly growing group living in neighborhoods where welfare dependency, female-headed families, male joblessness and dropping out of high school are so common that they appear to have become a way of life in these communities."

She estimated this underclass at about one million people in 1980, and said it is "predominantly minority, heavily concentrated in the large cities of the Northeast and Midwest, and very poorly educated." Another 1.5 million who are not below the poverty line live in similar conditions and "are at risk of being dragged into the underclass."

She recommended a multipronged attack on the underclass -- discouraging teen child-bearing, requiring absent fathers to support their children, encouraging welfare mothers to work, improving compensation of low-level jobs (perhaps through pay subsidies and tax credits), creation of "low-wage jobs of last resort" for the least-employable, and expansion of education and skills programs like Head Start, the Job Corps, Chapter I compensatory education and preschool programs.