The center-right’s Piñera coasts to victory on a pledge of market reform.

Chileans voted on Sunday in a runoff presidential election that was arguably the nation’s most important vote since the 1990 return to democracy. Center-right former President Sebastián Piñera, of the “Let’s Go Chile” coalition, ran on a platform to deepen market reforms and restore economic growth. Leftist Sen. Alejandro Guillier, candidate for the socialist “New Majority” coalition, promised to put social justice and government intervention in the economy above individual liberty. Ahead of the balloting Mr. Piñera—who got 36.6% of the vote in the first round against Mr. Guillier’s 22.7%—was narrowly favored. But pollsters said the race was too close to call. In the end Mr. Piñera won with some 54.5% of the vote; Mr. Guillier admitted it was “a tough defeat.” Yet the socialists gave Chileans who treasure freedom a scare. This election season ought to be seen as a shot across their bow. The failure of socialism in South America—particularly in Venezuela—is no secret. Nor is the success of the market economy in Chile, where the percentage of the population living in poverty fell to 11.7% in 2015 from 68% in 1990, according to the United Nations Development Programme. Yet Chile is increasingly polarized thanks to a public discourse dominated by the left. Chile’s right has spent more than three decades building institutions and refining policies in law and economics to create wealth. It imagined that the results of its work would be enough to build and sustain support.

But the socialists have been busy too. They have spent their time and money organizing factions and going to the streets to denounce, as unjust, a society in which some have more than others. Organized labor, especially in education, has been central to this strategy. This campaign season—including the strong showing of hard-left candidate Beatriz Sánchez in the first round—shows how successful it has been. Some months ago Mr. Piñera was heavily favored to win, in part because during his first term as president from 2010 to 2014 Chile’s gross domestic product grew at an average annual rate of 5.3%. Odds also were with him because of the record of Socialist President Michelle Bachelet, who has been in office since 2014 and currently heads the New Majority—which includes the Communist Party—that was backing Mr. Guillier. Ms. Bachelet is an avid admirer of the late Fidel Castro and the Cuban military dictatorship. She has spent her presidency working to dismantle Chile’s liberal economic model and using the bully pulpit to rail against private initiative.

Growth has suffered. GDP growth during her tenure, including the estimate for this year, is expected to average a paltry 1.8% annually.

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