As the campaign cycle enters a more serious phase, says Julian Zelizer, professor of history and public affairs at Princeton University, “Trump is going to need more than name-calling and scandal politics to overcome his lack of experience and worries that he’s not suited to the job.”

Mr. Trump’s appeal to working-class Americans rests on promises to wall off America from competition from foreign workers and goods. His line, “It’s not free trade, it’s stupid trade,” is hardly an invitation to a thoughtful debate. Yet it will be Mrs. Clinton’s challenge to counter by explaining her own evolving position on trade pacts, which has led her to oppose agreements she once supported. Going further, she can offer ways to assist workers who have been hurt by trade, as well as by general manufacturing job losses that have little to do with trade.

Mr. Trump’s opposition to trade deals is one element of a broader isolationism that can be discerned through the haze of his recent “America First” foreign policy address. In that speech, he threatened to walk away from various aspects of international engagement, from trade with China to NATO.

“We have not had a fundamental debate in a presidential campaign between American engagement in the world and isolationism since 1952, between Taft and Eisenhower,” says Max Boot, a conservative foreign policy analyst at the Council on Foreign Relations. “It’s easy to ridicule Trump’s cockamamie, half-baked proposals, and they ought to be ridiculed. But maybe it does force Clinton to articulate very clearly why we need to stay engaged in the world.”