They were the innocent bystanders of the "Crash of '87": the poor and lower middle-class, who watched in mid-October as stunned investors tallied their paper losses.

But in the weeks that followed, they also became casualties of Black Monday as they cut household budgets and delayed major purchases out of fear that the Oct. 19 stock plunge signaled bad times.

More than two out of every five Americans in households earning less than $20,000 a year said they had cut back personal spending due to worries about the economy after Oct. 19. But only one out of six persons with household incomes of $50,000 or more had reduced personal spending for that reason, according to the latest Washington Post-ABC News Poll.

That survey found that concerns about the economy have prompted almost one of every three Americans to cut back or postpone personal spending in recent weeks.

In the Washington area, people were even more likely to have cut back. A separate Post survey of local residents disclosed that two out of five -- 40 percent -- had changed the way they spend their money since the stock market plunge, and another 6 percent were seriously rethinking their spending plans.

The surveys document how closely public expectations of current and future economic performance -- whether those beliefs are accurate or not -- profoundly affect personal spending patterns.

Among the findings:

Slightly more than one out of eight Americans -- 13 percent -- have delayed or decided against buying a house, car, major appliance or some other costly item because of concerns about the economy. Locally, one of seven Washington area residents has postponed or opted not to make major purchases in the last eight weeks.

About one out of five persons nationally and in the Washington area has decided to spend less money on a major purchase due to worries about the economy.

One out of seven Americans has cut back on spending for entertainment and recreation. Locally, slightly more than one of five has cut back.

Seven percent of those in the national poll and 12 percent of Washington area residents have decided against taking out a loan, or decided to borrow less money in recent weeks.

One out of seven nationally and more than one of five in the Washington area has cut back on other types of personal spending.

A deeper look into the numbers disclosed that 32 percent of the 1,005 men and women interviewed had made at least one of the cutbacks mentioned in the survey due to concerns about the economy, according to the Post-ABC national survey.

And about one of five -- 19 percent -- had trimmed spending in more than one spending category since Oct. 19.

In the Washington area, 40 percent had reduced personal spending, and a quarter had cut back in more than one spending category.

Changes in spending plans are tied directly to perceptions about the overall economy. The surveys showed that pessimistic views of the economic future already have been translated into changes in spending patterns.

When asked if the drop in stock prices means the United States is going into an economic downturn, 43 percent of those surveyed nationally said it did while 52 percent disagreed.

And of those said the economy is headed for a fall, slightly less than half -- 46 percent -- said they had trimmed personal spending, while only 19 percent of the economic optimists had cut back.

Concerns about the economy have changed more than buying patterns. Significant numbers of Americans -- about one out of six -- said they intend to replace charge cards with cash on more purchases.

The local survey suggests a slight decline in Christmas spending by Washington area residents, but little if any decrease nationally.

When asked to compare their Christmas spending plans with last year, one out of three area residents said they would spend less, while about one of five said they planned to spend more. About two out of five of those surveyed said they planned to spend about the same amount.

Nationally, 25 percent said they would spend more and 29 percent said they planned to spend less. The remainder said they were spending about the same amount this Christmas season.

The Post-ABC survey results are consistent with other national polls completed in recent weeks. The Survey of Consumer Attitudes conducted by the University of Michigan in November also found eroding consumer confidence that researchers said meant more short-term bad news for business.

The Michigan poll found that the proportion of families that held favorable attitudes toward buying conditions for homes fell to 58 percent in November, down from 70 percent in October and 83 percent one year earlier.

But not all the news is bad, and not all the bad news is inevitable. "Although these trends indicate weakened sales prospects through early 1988, much will depend on the availability of price discounts and reduced interest rates, which consumers now expect in the upcoming months," said Richard Curtin, survey director for the Institute for Social Research at Michigan.

Figures are based on telephone interviews with 436 randomly selected District, suburban Maryland and Northern Virginia residents Dec. 10-12, and 1,005 adults nationally between Dec. 14-17. Margin of sampling error on the local survey was plus or minus 5 percentage points, and plus or minus 3 percentage points for the Post-ABC News national poll. Sampling error is only one of many potential sources of error in this or any other public opinion poll.

Polling analyst Kenneth E. John contributed to this study.