What is in store for Brazil-Argentina relations?

Argentina primary elections were established in 2009 as a way to limit the number of candidacies on the final presidential ballot, with tickets receiving less than 1.5 percent of votes being excluded from the race come October. At first, the primaries were used to decide which candidates would represent their parties, but now—with usually only one candidate per party, the PASO (Simultaneous and Mandatory Open Primaries) has become something of an election predictor.

On August 11, voters gave an astonishingly wide lead to the left-wing ticket, headed by Alberto Fernández and his running mate, Senator and former President Cristina Fernández de Kirchner. The duo got 47 percent of votes, against incumbent Mauricio Macri’s 32 percent. If this result is repeated in the final election, Mr. Fernández will win without the need for a runoff. As of now, he is already being treated as president-elect.

From this side of the border, Brazilian President Jair Bolsonaro has established Mr. Fernández and Ms. Kirchner as his political foes. Mr. Bolsonaro sought to stoke fears of a leftist win the day after the result, prophesying that their victory would turn the Argentinian-bordering state of Rio Grande do Sul into “a new Roraima,” invoking the specter of Venezuela’s economic collapse and the outpouring of Venezuelan migrants into Brazil’s northernmost states.

True to his style, Mr. Bolsonaro chose not to engage in geopolitical pragmatism. He could have defended the recently-signed Mercosur-EU trade deal (which was dubbed by...