For Republicans, the shift inward comes as their long dominance over issues of national security and international affairs is waning. Mr. Trump rejected Republican foreign policy orthodoxy but largely struggled to articulate a cohesive countervailing view beyond a vague notion of putting America first. He embraced strongmen, cast longtime allies as free riders and favored a transactional approach, rejecting any notion of the kind of values-driven foreign policy that had defined the party for decades. The party’s foreign policy establishment found itself exiled from Mr. Trump’s government and fighting for relevance against an insurgent isolationist party base. “To say that there is a single Republican foreign policy position is to miss what’s been happening within the conservative movement on these issue for the last 20 years,” said Lanhee Chen, a Hoover Institution scholar and policy adviser to a number of prominent Republican officials. “The characters change, the terminology changes, but the differences remain.”