Even Republicans hotly critical of Donald Trump’s brand of populist nationalism were a little giddy with excitement when he won the presidency with healthy Republican majorities in both houses of Congress.

Fond partisan fantasies of unified conservative government have given way to night sweats. Congressional Republican resistance to Mr. Trump’s new budget, which combines big increases in military spending with deep cuts to Medicaid and nutritional support for the poor, is one piece of evidence that the party’s legislative agenda is a mess.

The small-government fanatics of the Freedom Caucus and its former member Mick Mulvaney, now director of the Office of Management and Budget, may think huge tax cuts financed by reducing health benefits for millions of voters — 23 million if the American Health Care Act is enacted, according to the latest Congressional Budget Office analysis — is a great idea. But it’s wildly unpopular with the public, including Republican voters, many of whom rely on Medicaid. Moderate Republican legislators thus find themselves forced to pick their poison. Is it better to suffer electoral blowback for cutting popular programs or for fecklessly impotent unified Republican government?

Republicans need a fresh formula that can both win elections and govern. Mr. Trump’s nomination was a triumph of bread-and-circuses populism over reheated Reaganism. Yet the Tea Party wing of the Republican Party remains zealously committed to the idea that tax and spending cuts are the magic key to economic growth.