Newly minted California Gov. Gavin Newsom’s sweeping proposal to create a government-funded health-care system is unlikely to gain the needed support in Washington, but the request may be a way for Newsom and others to make the political case for states to move forward in areas where the federal government balks.

Newsom is appealing to President Donald Trump and congressional leaders for a “transformational cost and universal coverage waiver” that would allow California and other states to create their own health-care systems. The system, dubbed “single-payer health care,” would be financed by the government but delivered mainly through private providers....