The chairman of the House Committee on Aging assailed President Reagan's administration Monday for trying to "cripple" Medicare and Medicaid and for an "attitude of indifference" toward uninsured Americans.

Rep. Edward Roybal, D-Calif., made the comments in an open letter as he released a study that he said shows an "eight-year assault" on government health insurance programs for the elderly, the poor and the disabled.Addressing an issue Democrats have tried to exploit in the current political campaign, Roybal blamed "administrative mishandling, lackluster leadership and massive budget-cutting proposals."

"Contrary to the picture some would like to paint of an administration defending two critical health programs, Medicare and Medicaid, the truth is that the current administration did everything it could do to cripple both programs," he wrote. "This shows an eight-year attitude of indifference and inaction on uninsured and underinsured Americans."

The report drew information from the Health Care Financing Administration, the Congressional Budget Office and the General Accounting Office, Roybal said. It examined the impact of administration actions and legislative changes in Medicare, which provides health care payments for the elderly and disabled, and Medicaid, which is designed to help the poor.

"Time and time again, Congress rejected this administration's attacks and shielded the `health safety net' for millions of poor, disabled and elderly," Roybal said, noting the administration submitted to Congress a budget plan for fiscal 1987 that would have made a five-year cut in Medicare of $54.5 billion and in Medicaid of $18.1 billion.

Even with its defenders, however, Medicare was cut by as much as $35.9 billion from 1981 to 1987, the GAO found. Adjusted for inflation and removal of beneficiary premium increases, the congressional agency estimated Medicare still was reduced by $17.3 billion for the period 1980 to 1986.

Roybal said the health care cuts resulted in out-of-pocket costs for Medicare beneficiaries rising $8 billion more than explained by inflation alone during the 1980-86 period. The GAO estimated the costs were an inflation-adjusted increase of 42 percent for each of the 32 million Medicare beneficiaries.

"According to our own Committee on Aging study in 1986, these cuts, coupled with unrestrained price inflation for other health care, resulted in elderly out-of-pocket health costs rising 11/2 to two times faster than their incomes," Roybal said.