Flexing their muscle in the political battle over Obamacare repeal, a coalition of of hospital groups and systems sent a new letter to Congress dated Wednesday expressing “significant concerns” about the proposed GOP repeal and replace bill.

“It is likely to result in a substantial reduction in the number of Americans able to

buy affordable health insurance or maintain coverage under the Medicaid program,” they write in the letter. “We are very concerned that the draft legislative proposal being considered by the House committees could lead to tremendous instability for those seeking affordable coverage.”

The letter is signed by America’s Essential Hospitals, the American Hospital Association, the Association of American Medical Colleges, the Catholic Health Association of the United States, the Children’s Hospital Association, the Federation of American Hospitals, and the National Association of Psychiatric Health Systems—which collectively represent thousands of hospitals around the country.

The cite in their letter several areas of concern in the bill, particularly the provision that freezes the Medicaid expansion in 2020 and turns it into block grants parceled out to states—which experts say would reduce federal spending on the program by hundreds of millions of dollars.

“The proposed Medicaid program restructuring will result in both the loss of coverage for current enrollees as well as cuts to a program that provides

health care services for our most vulnerable populations, including children, the elderly and disabled,” they write.

The new letter follows on the heels of criticism from groups representing doctors, nurses, and the elderly.

In a separate letter this week, the American Hospital Association offered an additional warning about the GOP Medicaid plan: “If coverage is not maintained at the current level, those resources need to be returned to hospitals and health systems in order to provide services to what will likely be an increased number of uninsured Americans.”

The group also strongly disagreed with Republicans’ decision to move the bill forward without hearing first from the Congressional Budget Office, the non-partisan agency studying how much the bill will cost and how many people could lose their health insurance if it passes.

Read the full letter: