Miami-Dade's middle class is disappearing. That's the conclusion of a report released in June by the Washington, D.C.-based Brookings Institution.

The report -- "Growing the Middle Class: Connecting All Miami-Dade Residents to Economic Opportunity" -- notes that the county's middle-income residents have been steadily fleeing to Broward, Palm Beach and elsewhere in search of better jobs, lower housing costs and less-congested roadways. Meanwhile, Miami-Dade's poorest residents remain unable to climb the ladder of economic prosperity.

According to the report, almost 160,000 people -- most of them middle class -- moved away from Miami-Dade between 1990 and 2000. They were replaced with economic refugees, many illegal, who arrive with little education or training. Poor urban planning and a weak public transportation system, the report concludes, isolate many of the county's neediest residents in remote neighborhoods, where good jobs and educational opportunities are scarce.

The city of Miami, with 430,000 residents, is the poorest large city in the U.S. But poverty extends beyond the county's urban core. Miami-Dade, even with such ritzy enclaves as Coral Gables, Key Biscayne, Aventura and other upscale neighborhoods, is the nation's 12th-poorest county. Miami-Dade's median household income is $35,966, far below the national average of $41,994. The city of Miami's is a paltry $23,483.

Those statistics, coupled with the release of the Brookings Institution report, have prompted a group of business and civic leaders to draw up a blueprint for rebuilding and stabilizing Miami-Dade's middle class. That effort, the Community Prosperity Initiative, is focusing on providing low-income residents with better access to education and job training, improved public transportation and more affordable housing.

"Miami is incredibly polarized between the rich and the poor," says Eduardo Padron, president of Miami Dade College and co-chair of the Community Prosperity Initiative. "You see all this economic activity here, and yet such a large part of our population is not enjoying the fruits of that prosperity. Miami-Dade has so much potential, but without a solid, sustainable middle class, that potential will go unfulfilled," says Padron. "Our low-income residents need to know that they have opportunities and real access to these opportunities that are an alternative to flipping hamburgers the rest of their lives."