Like his onetime mentor, Jack Kemp, Representative Paul D. Ryan of Wisconsin leads the “happy warrior” wing of Republican conservatism. As House speaker, he wields more power than Mr. Kemp, a former lawmaker and vice-presidential nominee, ever did.

Yet an interview with Mr. Ryan suggests he’ll have trouble finding the decisive victories he craves anytime soon. The transformed landscape of 2016 keeps placing more obstacles in his path.

More than most politicians, Mr. Ryan holds fast to his political philosophy. It starts with lower tax rates and expanded trade to spur economic growth, revamped entitlement programs to curb government spending, and a streamlined safety net to increase incentives to work.

To achieve his vision, he argues, Americans must end the vexing period of divided government in which a Democratic White House clashes endlessly with a Republican Congress.