The Bush administration, continuing its fight to stop states from expanding the popular Children’s Health Insurance Program, has adopted new standards that would make it much more difficult for New York, California and others to extend coverage to children in middle-income families.

Administration officials outlined the new standards in a letter sent to state health officials on Friday evening, in the middle of a month-long Congressional recess. In interviews, they said the changes were aimed at returning the Children’s Health Insurance Program to its original focus on low-income children and to make sure the program did not become a substitute for private health coverage.

After learning of the new policy, some state officials said today that it could cripple their efforts to cover more children by imposing standards that could not be met.

Ann Clemency Kohler, deputy commissioner of human services in New Jersey, said: “We are horrified at the new federal policy. It will cause havoc with our program and could jeopardize coverage for thousands of children.”