Common currency between Brazil and Argentina a non-starter

Visiting Buenos Aires is a tradition for Brazilian leaders dating back to 1900, when then-President Campos Salles visited Julio Argentino Roca. On Thursday, Jair Bolsonaro became the 13th Brazilian president to pay a visit to our neighbors. The visit was marked by Mr. Bolsonaro’s breaches of protocol and unapologetic bravado.

International leaders usually avoid endorsing candidates in other countries’ elections—especially tight races—for one simple reason: they will have to deal with whoever wins. The Brazil-Argentina relationship is key to both countries, as our neighbors are Brazil’s third-biggest trading partners (trailing behind only China and the U.S.), and one of the main destinations of manufactured goods produced in Brazilian factories.

So conventional wisdom would suggest that a Brazilian president should be in good terms with whoever occupies the Casa Rosada. But, if Mr. Bolsonaro has shown us anything in his five months as president, it is that conventional wisdom doesn’t mean much to him. He has openly said he wants incumbent Mauricio Macri to win the October presidential race, telling Argentine newspaper La Nación that a return of Peronism would be a “major setback for both Argentina and South America.”

Then, Mr. Bolsonaro went further, talking to a crowd of business people from both countries about a common currency to be shared between Brazil and Argentina, which already has a name: the Peso...