President-elect Mauricio Macri’s promises to revitalize Argentina’s sagging economy with free-market reforms and improve strained relations with the United States resonated with voters, carrying him to a historic win that ended 12 years of often-conflictive rule by President Cristina Fernandez and her late husband.

But when the business-friendly opposition candidate takes office Dec. 10, he will inherit a country with around 30 percent inflation, near-zero economic growth and entrenched government social spending that private economists warn is not sustainable. He also lacks majorities in either chamber of Congress to pass his deep reforms.