BUENOS AIRES — Argentine voters sent the country’s presidential race into a runoff on Sunday, boosting hopes in the opposition after Mauricio Macri, the mayor of Buenos Aires, made a surprisingly strong showing against the candidate endorsed by President Cristina Fernández de Kirchner, according to partial results released by the electoral authorities.

Pointing to a remarkably tight race, Daniel Scioli, a former vice president supported by Mrs. Kirchner, took 35.7 percent of the vote with 84 percent of balloting stations tallied, against 35.3 percent for Mr. Macri. The votes obtained by Mr. Macri, a former president of one of Argentina’s most popular soccer clubs who ran to the right of his rivals, upend a race in which Mr. Scioli was viewed as a strong front-runner.

The candidates signaled an intense new phase of campaigning ahead of the runoff election on Nov. 22. Sergio Massa, a former ally of Mrs. Kirchner’s who moved into the opposition, could find himself in the role of kingmaker after securing 21 percent of the vote in the partial count, with analysts questioning whether he will forge an alliance with Mr. Macri.