Messrs. Trump and Johnson both also prevailed in part by moving away from traditional conservative economic orthodoxy. They juiced their policy offerings with promises of freer public spending to address middle- and working-class voters’ anger over the sacrifices they had been forced to make since the financial crash that began in 2007. That represents a significant departure from the positions their conservative forebears took when it came to government spending.

Speaking outside his official residence Friday, Mr. Johnson promised more spending on the nation’s public-health services, schools, policing and infrastructure. He said his government planned to unite the country and “level up” the less-advantaged.

“These conservative parties in the U.S. and U.K. are more and more throwing their lot in with working-class constituencies,” says Christopher DeMuth, a conservative scholar with the New York-based Hudson Institute who worked in the Reagan administration. “That’s a wonderful development. That is the future of conservative parties.”