Can Trumpism outlast Donald Trump? The selection of Senator JD Vance of Ohio as his running mate suggests that the answer is yes.

Even the most successful political parties are coalitions of odd bedfellows and competing interests. But parties risk decline if they cannot attract new voters or if they suppress internal debates. The Republican Party Mr. Trump inherited in 2016 had been defined by free market ideology and neoconservative foreign policy for a generation: From Ronald Reagan through both Bushes to Mitt Romney, mainstream conservatism pursued a narrow agenda of tax and entitlement cuts at home and wars of regime change abroad. Mr. Trump’s election was a clear if unexpected breaking of that mold. The only question was whether his legacy would dissipate whenever he eventually left politics or give birth to a true and lasting political movement.

Now we know the answer. The Republican Party we saw in Milwaukee this past week has been fundamentally changed. With Mr. Vance at his back, leading a new counterestablishment that Mr. Vance helped shape, Mr. Trump has cemented his legacy — and transformed the Republican Party.

Until recently, there was no counterestablishment in the Republican Party. There was merely a populist counterculture.

A counterestablishment is a government in waiting. Its goal is to win power through elections and then to staff government agencies with political appointees or career civil servants who have the expertise, discipline and bureaucratic skill to carry out a shared policy agenda.

Countercultures, whether of the right or the left, prefer performance to power. For some countercultural purists, protest is an end in itself. Working within the existing system is often seen as a betrayal of principle.