The study by the Urban Institute identified 20 congressional districts where more than 30 percent of the residents have no health care coverage. Eighteen of those districts were located in three states: California, Texas and Florida. In California, districts with the highest concentrations of uninsured were in Los Angeles and Orange County -- in areas heavily populated by immigrants and other low-income groups. Most of those areas are represented by Democrats who support the health care proposals -- proposals that promise significant new benefits to their constituents. But in three Florida districts where the uninsured account for nearly one-third of the population, Republican congressmen have maintained their opposition. Republican Rep. Mario Diaz-Balart, who represents portions of the southern tip of the state where 31 percent are uninsured, says that because none of the Democrats' proposed bills is expected to cover absolutely everybody, many of his constituents would fall between the cracks. 'My constituents, they also understand that they are the ones that get stuck with the bills,' Diaz-Balart said. Democratic Rep. Henry Cuellar, whose Texas border-region district ranks 10th on the uninsured list, says he's not yet ready to throw his support behind the Democrats' bills. Cuellar says he also represents high numbers of seniors, who fear cuts in Medicare will be used to pay for expanding coverage. In Massachusetts, the state with the lowest uninsured rate in the country, Democrats representing the 10 congressional districts there are considered guaranteed votes for whatever emerges as a final bill, despite the fact that the state's own version of health care reform -- which closely resembled the plans before Congress -- has done little to hold down health care costs for middle-class residents. A federal overhaul that mirrors the Massachusetts approach probably would leave many of these voters' existing insurance unchanged while adding new taxes to help subsidize the uninsured. Democrat Joe Sestak, who represents an affluent district that includes portions of the Philadelphia suburbs and ranks among the highest insured districts outside of Massachusetts, is one of the members who helped craft the health care bills in the House. 'Do I have some (constituents) that are really upset and opposed? Yes, I do,' Sestak said. But Sestak says he believes his district will benefit in the long run if the country addresses the strains on the health care system caused by the costs of the uninsured. 'This is one of those times where what's the right thing to do happens to coincide with an economic requirement to do this,' Sestak said. 