Facing a competitive reelection campaign, Argentine President Mauricio Macri took an unexpected gamble last month in his choice of a running mate: Miguel Angel Pichetto, an opposition stalwart who has nonetheless helped the government advance critical reforms from his perch as the most senior senator from the Justicialist Party, the main political vehicle for the opposition Peronist movement.

The move was widely praised by analysts. Pichetto is a moderate, so he can help Macri lure Peronists who are anxious about their party’s more populist ticket, which includes the polarizing former President Cristina Fernandez de Kirchner as its vice-presidential candidate. ...