They agree on trade, but the ticket’s No. 2 marks a major shift on economic policy.

On the surface, this GOP convention is the most ebullient and unified in decades. Beneath it, many elected officials are deeply uneasy. The source of that agitation is Donald Trump’s running mate. At a glance, Mr. Trump’s choice of Sen. J.D. Vance makes sense, as he appears to be a prototype of Mr. Trump. Protectionist. Passionate. America First. But he isn’t Mr. Trump—not at all—in ways that are hugely consequential for the conservative movement and its future. In choosing Mr. Vance now, Mr. Trump sets him up for 2028, which means a monumental internal fight is coming. We’ll soon find out how much control Mr. Trump really has—and intends to keep—over his party. To listen to the convention speeches and the on-the-record GOP chatter, Mr. Vance is a perfect running mate for a candidate laser-focused on winning this election with working-class votes. He’s right there with Mr. Trump on tariffs and the border. He talks passionately on inflation. He connects to the workingman, whom he delights by skewering the media and the Biden administration. That’s where the similarities end. Mr. Vance’s bestselling 2016 memoir, “Hillbilly Elegy,” was mostly a social and cultural critique. His later political opinions initially sounded distinctly Democratic. He reveled for a time as a Never Trumper, with a 2016 essay in the Atlantic, “Opioid of the Masses,” in which he explained, “Donald Trump feels good, but he can’t fix America’s growing social and cultural crisis.” He now says he was wrong about Mr. Trump, but his record since joining the Senate last year suggests he still rejects much of Mr. Trump’s economic agenda.

Mr. Vance believes government is needed to right the problems in his hometown. He’s OK with raising the minimum wage to $20. (Even Bernie Sanders is asking for only $17.) He bashes banks and thinks Federal Trade Commission head Lina Khan is “doing a pretty good job.” He wants government to direct industrial policy, with tax credits for favored industries. He’s “not philosophically against raising taxes on anybody.” Maybe because he wants more welfare for low-income households. He and his “national conservative” allies may call this populism, but Mr. Trump’s successful Republican formula couldn’t be more different. The former president has certainly shifted the party, but as Ohio Rep. Jim Jordan put it recently, he has made the GOP into “a populist party rooted in conservative principle.” A businessman at his core, knowledgeable about markets, an invoker of Reagan, Mr. Trump’s first-term success was anchored in the core beliefs of limited government, more freedom, trust in providing opportunity to the American people. His tax cuts, impressive deregulation and handcuffs on the bureaucracy worked magic. One instance of the Grand Canyon between the two men: Mr. Trump’s belief in workers vs. Mr. Vance’s skepticism of right-to-work laws, which empower workers who don’t want to join unions.

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