WASHINGTON — Congressional Republicans, after years of blasting the Affordable Care Act for disrupting Americans’ health care, are pushing changes that not only would strip health coverage from millions but threaten to upend insurance markets, cripple state budgets and drive medical clinics and hospitals to the breaking point. President Donald Trump and GOP leaders have touted their proposals to repeal the health care law — one passed by the House last month and a Senate version unveiled last week — as a critical fix to the Affordable Care Act. But in physicians’ offices and medical centers, in state capitols and corporate offices, fears are growing that the unprecedented cuts proposed in the GOP legislation would create even larger problems in the health care system. “These reductions are going to wreak havoc,” said Tom Priselac, chief executive of Cedars Sinai Health System in Los Angeles, one of the country’s leading medical centers.