While running for governor, however, Mr. Gianforte switched course and said that he would not support repeal — an acknowledgment of how popular the law is with the 130,000 Montanans covered by expanded Medicaid and the insurance exchange.

Celeste Thompson, a home health aide and activist in Missoula, is one such Montanan. After the state passed Medicaid expansion under the A.C.A. in 2015, she told me, she was able to buy eyeglasses for the first time in years. Before that, Ms. Thompson had relied on the hospital emergency room and charity-care programs for everything from checkups to cancer treatment.

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