PRESIDENT CARTER wants his forthcoming urban-aid program to embrace all "urban areas," not just the hard-pressed cities of the Northeast and Midwest. That brings in suburbs, smaller towns and growing Sun Belt cities whose problems are less acute. Apparently Mr. Carter has made the political judgement that only an inclusive approach can gain enough congressional and public support. True, this should reduce regional rivalries, competition between cities and suburbs, and the like. The danger is that the administration will wind up with an urban policy that lacks either focus or force.

According to the policy outline the President approved the other day, his new and revised urban programs are going to concentrate on people and places "in distress." However, although aiding needy people wherever they may live is good and vital, it is not the whole of a real urban policy. If one of Mr. Carter's aim is to help revive communities, limited federal resources should be concentrated on the cities whose economic, fiscal and social problems - and their human costs - are most acute.

Wide dispersal of economic development aid, which Mr. Carter intends to emphasize, could have doubly harmful effects. The communities best able to stimulate private investment may well be those with lower taxes, better public facilities and more skilled labor - is short, those less distressed. Offering federal incentives "even handedly" could accelerate the drain of private from worse-off cities, leaving them poorer than before.

A broadly based approach does have potential benefits. It could encourage healthier communities to ward off problems by conversing aging neighborhoods and moblizing private resources to train and employ poor youth. The administration is already using community-development grants to push suburbs to carry out subsidized-housing plans. New programs could provide more leverage for opening up job and housing opportunities and encouraging more regional sharing of urban burdens and costs. Finally, by backing an economic-development program with widespread appeal, Mr. Carter might be able to win more support for other efforts that help urban areas, such as tax incentives for in-town investment, and-yes-more fiscal relisf for hard-pressed cities, starting with New York.

But to realize these benefits, Mr. Carter will have to make demands on the very constituencies he is trying to court. He will have to ask the private sector to invest much more in area and people that have been regarded as bad risks. He will have to challenge suburban exclusiveness. And he will appeal effectively not just to American's community spirit, but also to their good will and generosity. Much depends on program details that are still being hashed out. Even more will depends on Mr. Carter's readiness to make aiding cities not just a nationwide effort, but a major national commitment.