Community health centers, pioneered by Boston almost a half century ago, are a cornerstone of efforts to expand healthcare coverage to all Americans while cutting costs. By offering preventive care to poor, uninsured residents, these neighborhood centers reduce the chances these patients will end up in hospital emergency rooms for vastly more expensive treatment, with taxpayers picking up the bill, health analysts say. However, a funding battle in Congress threatens to choke off the services. Republicans want to cut more than a billion dollars bound for such clinics, a move that would force centers across the country to cut back many services. Dozens of new centers could be forced to close, advocates said.