The Obama administration is considering slashing support for Massachusetts hospitals, members of the state Congressional delegation said Monday, offering a window into secret negotiations that could affect the state's groundbreaking health care reform efforts and some of its major employers. In a letter sent to Kathleen Sebelius, the president's top health and human services adviser, and Donald Berwick, who runs the federal Medicare and Medicaid programs, Sens. John Kerry and Scott Brown and the 10 members of the Massachusetts U.S. House delegation warned that a change in support proposed by the Obama administration "could significantly threaten payments to Massachusetts health care providers providing care for the most vulnerable populations."