Just a few weeks ago, Republican elites were falling in line behind Donald Trump following his victory in the presidential primaries over his establishment challengers. Now, even many Republicans who, against their better instincts, endorsed the tycoon are expressing public unease over his divisive and undisciplined campaign. Meanwhile, Bernie Sanders at last has ended his guerrilla war against Hillary Clinton, but is now holding her campaign hostage to a list of policy demands that would pull her from the wide-open center ground of American politics to the leftist fringe.

Many Americans in the middle of the political spectrum feel that they have no one to vote for in November. Meanwhile, many allies overseas are bracing themselves for radical changes to American foreign policy that could turn the United States from a stabilizing to a destabilizing force in world affairs.

The principal problem is on the Republican side. From the perspective of its establishment elites, Trump has conducted a hostile takeover of the party. Republicans have traditionally advocated policies that promote free markets, free people, and an internationalist foreign policy. But Trump supports economic protectionism, illiberal controls on immigration, an ethnic majoritarianism alien to the American tradition, and an approach to foreign policy that seems to reserve more vitriol for U.S. allies than adversaries...