AS THE NATION'S cities continue to decay, urban mayors complain that their cries for help are being ignored. State and federal governments have little money to devote to cities. Even if they did, few citizens outside the affected areas would be likely to view such spending favorably. Instead, politicians are focusing on the predominantly suburban middle class, many of whom feel squeezed by the recent economic downturn. This new, strategy makes sense. According to 1990 Census figures, 48 percent of Americans now live in the suburbs while 29 percent live in urban areas. Forty years ago, the numbers were 26 percent and 35 percent respectively.