Despite seven years of economic expansion, the percentage of Americans in poverty remained stubbornly high at 12.8 percent last year, the government reported yesterday.

If the recession that many economists expect over the next few months follows the patterns of the recent past, millions more Americans could be pushed below the poverty line, boosting the rates higher than at any time since the mid-1960s.

Currently 31.5 million Americans live in poverty, according to an annual study released by the Census Bureau. Though down several points from the end of the 1981-82 recession, the poverty rate was virtually unchanged from the 1988 figure of 13 percent and is higher than at any time during the 1970s.

"With the economy now running out of steam and a new recession threatening, the poverty rate is likely to start rising again," said Robert Greenstein, director of the Center on Budget and Policy Priorities. "If a deep recession sets in, {the poverty rate} could reach levels not seen in a quarter century."

In every recession for the past 25 years, the pattern has been clear: When unemployment goes up, poverty goes up. But when the nation entered the recession of the early 1980s, the poverty rate was 11.7 percent, substantially lower than the current rate. By 1983 poverty had risen to 15.2 percent.

The government defines poverty as cash income below $9,885 for a family of three and $12,675 for a family of four.

Economists across the political spectrum said yesterday the current economic picture could mean an even greater rise in poverty.

"It means that poverty rates have more or less bottomed out at a very high level," said Isabel Sawhill of the Urban Institute. "From here on out they are likely to shoot up."

"It doesn't look like it's going to be a very good couple of years," said Henry Aaron of the Brookings Institution. "The economy is not going to be kind."

Aaron said recessions have historically taken a larger income bite from the poor and that, given current budget pressures, government relief will be unlikely. At the same time, the rise in oil prices will place even greater pressure on heating and transportation costs.

The Census Bureau also found that disparities in income between the rich and the poor have been sharply increasing over the past 20 years. In 1969, the poorest fifth of households received 4.1 percent of all household income, compared with 3.8 percent today. Over the same period, however, the richest fifth's share of all household income rose from 43 percent to 46.8 percent.

Many economists attribute this to changes in benefits programs and tax policy. But both the persistence of a relatively high rate of poverty and income disparities are also attributed to demographic and economic changes.

Daniel Weinberg, chief of the Census Bureau's Housing and Economic Statistics Division, said one factor is the growth in the number of female-headed families, particularly those with children. Poverty rates for such families are 42.8 percent.

Greenstein cited a flattening out of average wages, which have fallen since the 1970s after accounting for inflation and failure of welfare payments for the lowest income families with children to keep pace with inflation.

Yesterday's report also showed that measured in constant 1989 dollars, median household income rose from $28,537 in 1988 to $28,906 in 1989, with blacks at $18,083 per household slightly below Hispanics ($21,921) and both far below whites ($30,406).

The same racial disparity emerged in the poverty figures. The proportion of whites in poverty was 10 percent, of blacks 30.7 percent and of Hispanic-origin, 26.2 percent.

The figures also show that the poverty rate for children under 18 was 19.6 percent, the highest of any age group; for the elderly, the rate was 11.4 percent. Once the elderly were the poorest, but Social Security and pensions have greatly improved their income levels in recent decades.

Some analysts believe that income at the poverty line does not provide a family with sufficient resources to make ends meet. Others, including the Heritage Foundation, contend the current system overstates poverty because it does not count non-cash benefits such as food stamps and free medical care as income.

"We should be concerned about those people who fall into material deprivation through no fault of their own," said Robert Rector, a policy analyst at the foundation. "But these numbers simply blur the picture."

In a series of experimental calculations, the Census Bureau found that the poverty rate would be only 10.4 percent if most of the alleged faults of the current system were corrected. This means starting with after-tax income (the current measure starts with before-tax income, which some criticize as unfair since only after-tax income is available for a person's use), counting non-cash benefits such as food stamps, Medicare, Medicaid, housing aid, the value of employer-provided health insurance and the like.