Decades of middle-class flight have left many American cities impoverished, crime-ridden and dominated by minorities and immigrants. As jobs and investment funds have migrated to the suburbs, many urban areas have become monuments to unplanned, unsightly sprawl. Such entities, critics say, are geographically inefficient and economically unstable. A growing movement of planners and downtown boosters is exploring new ways to fill in the urban voids by tapping the resources of regionwide cooperation. Forging city-suburban partnerships to better manage limited funds is an attractive theory in today's climate of shrinking federal support. But implementing it may invite opposition from politically powerful suburbs reluctant to pour money into cities some view as beyond help.