With a sphinxlike smile, Mr. Macri deflected questions about the depth of the adjustments to economic policy he plans to make if he wins the election on Sunday.

This scene on Thursday would have been nearly unthinkable just a month ago, when Daniel Scioli, President Cristina Fernández de Kirchner’s preferred successor, was expected to glide into the presidency. But after mounting a frenetic campaign that crisscrossed Argentina and pushed the race into a runoff, taking him into what were believed to be Mrs. Kirchner’s strongholds, Mr. Macri is emerging as the front-runner.

“Everything is possible in politics, especially when those in power wield their influence with arrogance,” said Aída Villalpando, 49, a community activist who traveled from the provincial capital, San Salvador de Jujuy, to show support for Mr. Macri, the mayor of Buenos Aires. Ms. Villalpando said Mrs. Kirchner’s supporters had intimidated voters, and she cited their actions as among the main reasons she planned to vote for Mr. Macri.

With polls showing Mr. Macri in the lead, the possibility that he could win has many gauging the impact of such a shift in Argentina. Since 2003, the country has been under the sway of Mrs. Kirchner, one of its strongest leaders in decades, and her husband, Néstor Kirchner, who died in 2010. Under them, Argentina recovered from an economic crisis as the authorities asserted more control over the economy, deepened energy subsidies and clashed with the news media.

Mr. Macri, 56, has vowed to roll back such policies, pointing to galloping inflation and intensifying political divisiveness, while running a largely nonconfrontational campaign seeking to soften his patrician image as the scion of a wealthy Argentine family.

His opponent, Mr. Scioli, 58, a former speedboat racer and the departing governor of Buenos Aires Province, has increasingly gone on the offensive, calling Mr. Macri “a snob from Barrio Parque,” one of the most exclusive neighborhoods in Buenos Aires.

Mr. Scioli accuses Mr. Macri of concealing plans to put in place market-oriented policies, which many Argentines blame for an economic crisis in 2001 and 2002, and of cozying up to international financial institutions from which Mrs. Kirchner has distanced the country. “Don’t let them fool us under the slogan of ‘change,’ ” Mr. Scioli said Thursday in a closing campaign speech. “Because it’s a lie; it’s structural adjustment and a pact with the International Monetary Fund.”

Stunning a political establishment that had grown accustomed in recent years to blistering attacks on critics by Mrs. Kirchner and her supporters, Mr. Macri has largely avoided such tactics during his campaign. In an interview with journalists here in Jujuy Province on Thursday, he spoke in a low monotone.

“I’ll have a balanced economic cabinet,” Mr. Macri said of his choices for top economic policy posts, declining to say who would fill them while drawing a contrast with previous governments on the left and the right in which star economists have wielded substantial power over other ministers.

Dressed in a blue shirt without a tie, Mr. Macri greeted other questions with the barest of smiles while sitting in a hotel lobby. At the end of a long campaign, he seemed neither exhausted nor dazed, resembling instead someone who might have emerged from a session of meditation.

Hewing to his plan to restore investor confidence in the economy, Mr. Macri calmly criticized what he views as the unreliability of official statistics in Argentina. He avoided mentioning Mrs. Kirchner or Mr. Scioli by name. Touching for a moment on a politically delicate subject, he said he planned to review a deal with China to build a nuclear reactor in Argentina.

Some political analysts question how a Macri presidency would deal with Peronism, the ideologically diverse political grouping that has long dominated Argentine politics. Still, other analysts say that Mr. Macri has made strides in reshaping Argentina’s political landscape, especially with the victory of a rising star in his party, María Eugenia Vidal, who was recently elected to the coveted governorship of Buenos Aires Province, a Peronist stronghold. Ms. Vidal’s triumph, they said, has lifted a veil of doubt about Mr. Macri’s ability to govern beyond the city of Buenos Aires, a pocket of relative prosperity enclosed by poorer sprawl not covered by his administration. Those surrounding areas are home to nearly a quarter of  Argentina’s population. Crucially, mayoral races across the province were also won by Mr. Macri’s candidates, easing fears of struggles between Ms. Vidal and influential Peronist power brokers.

“The geography of Argentine politics has changed,” said Julio Burdman, an Argentine politics professor.

Some are even suggesting that the only issue at play on Sunday is Mr. Macri’s margin of victory. A poll by Elypsis, a political research firm thataccurately predicted the first-round result, gave Mr. Macri 47 percent of the vote, against 39 percent for Mr. Scioli, with 11 percent undecided. The survey, taken on Nov. 16 and 17 in interviews with 2,200 people around the country, had a margin of sampling error of plus or minus three percentage points.

A small margin of victory may not be enough to give Mr. Macri a mandate,wrote Carlos Pagni, a political commentator, in the newspaper La Nación. “Macri needs a conclusive victory to carry out reforms that would allow him to relaunch the economy,” Mr. Pagni wrote.

Still, others questioned the reliability of the polls when so many failed to predict Mr. Macri’s strong showing in the first round. The race remained volatile even after the candidates formally wrapped up their campaigning on Thursday, with Mr. Scioli contending that he had the support of Pope Francis, who is from Argentina and ranks among the country’s most popular figures. Francis has urged his countrymen “to vote according to their conscience.”

Jaime Durán Barba, an Ecuadorean campaign strategist for Mr. Macri, disputed Francis’ importance, speaking in favor of decriminalizing abortion and telling reporters on the sidelines of Mr. Macri’s coca-chewing ritual that “a pope doesn’t influence more than 10 votes in a country.”

Mr. Macri reacted on Twitter, saying Mr. Durán Barba’s statements “do not represent my thinking.”