The basic question about President Carter's new national urban policy is whether it can really save decaying central cities that are losing people, business and local tax revenues.

The answer is almost certainly not in the short run.But it might help in the long run.

Carter's program contains all sorts of inducements aimed at influencing people to live and raise families in cities, and companies to locate their plants there.

But administration officials admit that even if all the programs were approved by Congress -- which is doubtful -- they would make a difference only in marginal cases.

The family deciding where to live and the developer deciding where to locate a project are still going to consider such things as land and building costs, crime and whether the schools are any good.

Even with Carter's incentives, they may still decide a city is not worth the hassle.

Housing and Urban Development Secretary Patricia Roberts Harris noted the other day that in the last 15 years, 150,000 people moved out of Chicago and 140,000 moved in.

"It's a net loss of 10,000," she said. "But those who left were the best educated, the higher-tax-paying people, and those who came in tended to be the more dependent population. So the loss is really a great deal more than just the numbers."

Pollster Louis Harris found in a recent survey that 47 percent of people under 30, particularly those who children, were thinking of moving out of cities. "The key is public schools," he said.

And a survey of 352 major corporations found their commitment of the cities was, in many cases, less than enthusiastic.

Thus, the problem is enormous, and in money terms Carter has chosen to push for the expected quick fixes of big sums -- money for more jobs, especially for hard-to-employ young people, and continued antirecession funds that would go only to communities with high jobless rates and would no longer be shared with states.

He has also opted for quick fixes of the small sums -- a little extra to spruce up neighborhoods, a little extra to fight crime, more money for health and social services and city transporation.

But it should be noted that in actual outlays, the urban program would mean only an extra $742 million in fiscal 1979, which begins Oct. 1, and $2.9 billion in fiscal 1980. Even throwing in the $1.7 billion in annual tax incentives that Carter wants to give businesses that invest in cities or hire young people, the extra aid totals would be $2.4 billion in 1979 and $4.6 billion in 1980.

These figures are relatively small when measured against the $30 billion to $40 billion that localities already receive in federal aid each year.

The administration recognizes that, as a White House document says, "there is a limit to what the federal government can accomplish alone -- there is a limit to what the federal government can accomplish alone -- there is a limit to what even all public-sector funds can do, unless their use is designed to induce significant reinvestment by the private sector in urban areas."

And that is where the problem of attitudes comes in.

Perhaps the long-range value of the Carter plan will be its effort to change the focus of many government programs that in the past have encouraged, even subsidized, sprawl, and to direct them to favor more concentrated development.

For instance, the government for years has been putting federal buildings in suburbs and exurbs. Now it's going to put them in cities, Crter says.

For years it has brought goods and services from suppliers regardless of where they are located. Now it's going to concentrate in areas of high unemployment, according to Carter. And it plans to triple its procurement -- to $3.5 billion -- from minority-owned firms in the next two years.

For years HUD has offered mortgage money for middle-income apartment projects built anywhere, and almost all the construction it fostered was in the suburbs. Now the 7.5 percent mortage money will be available only to developers who locate in cities that are considered "distressed."

Since 1973 the Environmental Protection Agency has given far more waste water treatment grants to smaller communities than to big cities -- a program that Massachusetts Gov. Michael S. Dukakis has called "sewering cornfields." Now EPA, though constrained by a congressional manadate that says states must set priorities, plans to limit construction of new sewers and spend more money fixing up old sewer systems.

Since the 1960s the Economic Development Administration of the Commerce Department has given water and sewer grnts and business loans to develop rural areas. Now it plans to spend half its money in cities.

For decades the government has been building 42,000 miles of federal interstate highways, vitally aiding the commerce of the country but taking jobs, revenues and people out of cities at the same time. Now the Department of Transportation is shifting gears to stress rebuilding existing roads and bridges, most of them in cities.

Will any of these steps significantly affect the quality of life and the fiscal health of cities?

Administration officals insist they might. HUD SEcretary Harris argues that for the long haul the "single most important decision we've made is to study the impact that new federal activities will have on urban areas. At the very least, we hope we can avoid the negative impacts from now on."