Metropolitan Washington indeed does continue to be "an island" in America, as Jimmy Carter said one week ago tonight in attempting to set himself apart from the city in which his government is based.

Rhetoric aside, the island is one of unusual urban affluence for most of its 3 million residents.

New evidence of Washington's economic strength is due to be published tomorrow by Sales and Marketing Management magazine in its annual national survey of American consumers' buying power for 1978.

There is nothing in this year's survey to require a change in perception about the relative wealth of people who live and work here, whether or not they happen to be employed (at least for now) by the federal government that Carter heads.

As in other recent years, no other large population base in this country centered around one city has as much money to spend per household as does that of metropolitan Washington. As in the past, this average affluence masks the poverty of many families, particularly in the District and some of the older suburbs. The average figures also don't reflect the millionaire-status of other area residents.

But the new survey of income does come at a particularly interesting time because Washington-area consumers and businesses are tightening their spending habits in anticipation of recession. Moreover, recent population trends here and an inclination of many residents to halt or slow the economic development that creates new jobs, raises a question about how long metropolitan Washington will rank as No. 1 in the affluence race.

For now, however, there is no contest.

Per-household income in the D.C. area, after taxes, was $24,806 in 1978, 32 1/2 percent above the U.S. average and surpassing every other large city area among the 10 largest. Detroit was second with $23,623, and New York was 10th at $19,090.

Only if the combination of Nassau and Suffolk counties on Long Island is considered a separate "metropolitan area," with no central city and an urban magnet that is really New York, is Washington eclipsed in spendable income per-household, because the Nassau-Suffolk figure is $25,394.

Total spendable income in this area last year was $26.7 billion, up $2.2 billion, or 9 percent, from 1977. Retail sales totaled $12.5 billion, up $1.4 billion, or 13 percent, from 1977. Per-household sales of $11,624 were 9.4 percent above the national average.

Washingtonians also spend a lot of money at drug stores, bars and restaurants - more per capita than any other Americans. A Commerce Department report last week showed that restaurants and bars have become the biggest retail business in D.C., and the Sales and Marketing Management study tomorrow will report that spending for eating and drinking establishments throughout the area rose 172 percent in the past decade to $1.3 billion in 1978.

Drug store sales rose 99 percent over the same period to $603 million, with Washington also No. 1 in per-household purchases.

Automotives sales totaled $2.4 billion in the area last year, up 145 percent in the past decade and the fourth largest per-household volume of the 10 largest markets. Food store sales were $2.4 billion, up 112 percent but only eighth highest in per-household comparisons.

Furniture sales at $642 million last year put Washington as fourth highest in per-household purchases, reflecting the growth here of new household formations by young people, and general merchandise sales totaled $1.7 billion, also fourth highest in the per-household comparisons among the 10 largest markets.

The number of area households has increased by 154,400, or 17 percent, since the end of 1970 and now totals 1.08 million, according to Sales and Marketing Management data. At the same time, according to various studies, there has been a decrease in the average size per household, a high rate of increase in the number of household relative to the rate of population, an increase in the proportion of women in the work force and a growth of single-person households.

While average spendable income per household rose 83 percent to $24,806 in 1978 from $13,539 in 1970, area consumer prices went up 67 1/2 percent.

In average spendable income per household, Fairfax County ($30,169) and Montgomery County ($27,958) were ranked first and second among local jurisdiction in 1978. The rate of growth, both in households and income, has been fairly even for the Maryland and Virginia portions of the area as a whole, but the largest relative gains have been made in three more rural counties: Prince William, Loudoun and Charles.

These three counties, although part of the standard metropolitan area measurements and closely tied to Washington by economic trends, recently were cited by the Center for Municipal and Metropolitan Research as among the "corridor-fringe" jurisdictions of the greater Washington area - where population has increased during a period when closer-in population growth has stagnated.

The outward thrust of population growth was cited in the report by center Executive Vice President Atlee Shidler as a complication for the metropolitan area in solving its problems - particularly those of economic growth and mass transit.

Indeed, the golden era that has run through the 1960s and the 1970s could be concluded before the next decade ends if economic stagnation sets in, helped along by population expansion in distant Maryland, Virginia and West Virginia counties while the metropolitan areas holds steady and few new jobs are created.

Concerned about the absence of adequate jobs in the future to support a viable economy in all jurisdictions of the broader region, Baltimore and Washington business leaders are forming a "common market" regional promotion program to address this very issue.

Worrisome trends include:

Consumer prices so high in the immediate D.C. area that outsiders don't want to relocate here.

Sewer moratoriums on construction that reflect refusal of area governments to cooperate in meeting a regional requirement.

Unequal education opportunities with deteriorating public schools in some communities and excellent public schools elsewhere, and larger enrollments for private schools that are seen as an "escape" but which have uneven qualities, too. Overall, there is no evident attempt to develop any sort of city-suburban cooperation in seeking educational excellence for all.

A mismatch of the labor force and job opportunities, reflected in the bleak faces of unemployed black young people in D.C., while jobs go begging at distant locations that are hours away by bus.

Continued weakness of the D.C. economy. The District is the urban centerpiece for the region, but it has an inadequate tax base and a substantial number of commuting workers who don't want to pay income taxes in the city - even though they would be required to do so in most similar interstate commuting situations elsewhere in the nation.

And, apparently, it is unlikely that a Carter administration so interested in setting itself apart from Washington will use any muscle to deal with purely local problems. This disinterest is reflected in White House opposition to Capitol Hill legislation providing long-term subway financing. Also, the Executive Branch hasn't challenged random cutting of the D.C. budget or a sharp cutback in the federal payment to the city, also at the hands of Congress.

Symbolic of this new approach to Washington problems, some of those beautiful flower gardens that were planted by Lady Bird Johnson have gone to rot.

The problem of government neglect is not confined to the District, moreover, as Northern Virginia citizens found recently when they tried to get the attention of Gov. John Dalton in far-away Richmond for what was a regional gasoline shortage.

Dalton's office might be interested in the Sales and Marketing Management figures, which show that not only do Northern Virginians make up 20 percent of the state's households but also they account for 30 percent of after-tax income in the state and 24 percent of all Virginia retail sales.

There is a similar situation in Maryland, with the D.C. suburbs accounting for 31 percent of all households in the state, 39 percent of after-tax incomes, and 38 percent of retail sales in the state.

A lack of interest or disdain on Capitol Hill and at the White House, but a desire to continue holding D.C. purse strings. Perceptions of the Washington area as somehow remote from the rest of Virginia and Maryland in the government offices of Richmond and Annapolis. A complex region with many local governments and as many ideas about future growth. Conflicting attitudes about what area residents want.

No wonder that Shidler concluded in his report that "Washingtonians seem to be numbered among those metropolitan Americans with the least coherence or consistency in their views about the future of the region."

Noting the apparent conflicting desires for lower taxes and high-quality public services, an assured water supply in summer and lower health care costs, local government self-sufficiency and a reluctance to pool any authority in the interests of economy or efficiency, Shidler stated:

"The people of Washington apparently want a large amount of fixed-rail transit, they want cleaner water and cleaner air, they want good affordable housing for their baby-boom children come of age, they want an adequate energy supply, they want to reduce youth employment and they want to save neighborhood schools.

"They seem not, however, to want the kind, amount or location of either residential or commercial and industrial development that probably is necessary to achieve these goals with a reasonable degree of effectiveness at a reasonable cost."

It would be no special loss if, as a result of these contradictory goals, the Washington area loses its top rank as the nation's most affluent market in terms of per-household money to spend. Houston is moving up fast, so why not let the Texans take the title away? Being labeled as "most affluent" doesn't add much glory to an urban area and helps breed the sort of anti-Washington attitude expressed by the current White House resident.

But that is not the real problem. If economic expansion slows to a crawl and pockets of unemployment grow, warfare between area jurisdictions for remaining jobs would only be exacerbated and the seeds for social crisis would be nurtured. That would create an "island" in the American landscape, surrounding the national capital, that would worry even Jimmy Carter. CAPTION: Graph, Population and Income Compared, By Dave Cook - The Washington Post