Mexico entered a brave old world on Sunday by electing former Mexico City Mayor Andrés Manuel López Obrador to a six-year term as president. The 64-year-old left-wing populist has lost twice, in 2006 and 2012, but this time he won against two weak candidates while promising more moderate policies.

AMLO, as Mr. López Obrador is known, is a career politician from the oil-rich southern state of Tabasco. But in this race he ran as an outsider, pledging to end corruption, upend the establishment in politics and economics—which he called “mafias of power”—and use subsidies to lift the poor, especially in rural areas. He said he would make Mexicans better off by assigning the state a larger role in the economy.

AMLO has moderated his rhetoric from the decades when he railed against private investment in energy and other economic competition as Mexico shifted toward free markets. But his Morena party, which he founded in 2014, is home to many who are nostalgic for the not-so-glorious days of Institutional Revolutionary Party (PRI) dominance. They have never accepted the country’s transition to an open economy with more political and institutional transparency.

Mr. López Obrador’s challenge will be keeping those supporters happy without derailing Mexico’s economic progress under the North American Free Trade Agreement. Except for some of Mexico’s crony capitalists, entrepreneurial Mexico largely opposed his candidacy.

The most extreme fear is that AMLO will attempt to consolidate power a la Hugo Chávez in Venezuela. But Mexico has made considerable economic and political progress in 30 years of reform presidencies. The Mexican government doesn’t hold the oil power that Chávez had, and a Mexican president is constrained by his country’s obligations under NAFTA—unless President Trump tears it up. Financial markets will also vote on AMLO’s government each day through foreign-exchange markets and the value of the peso.

The U.S. can help by completing a renegotiated NAFTA deal as soon as possible. Mr. Trump’s anti-Mexico rhetoric played into the hands of Mr. López Obrador’s left-wing nationalism, but troubles in Mexico will inevitably spill across the border—wall or no wall.

AMLO’s anti-corruption pledge will also be tested by how he handles the country’s already weak rule of law. He has said he will amnesty drug traffickers and not investigate corrupt politicians for past crimes. His government’s cooperation with the U.S. on regional drug and immigration enforcement will also be important to watch.

This election has dramatically altered Mexican politics. The once-dominant PRI is now a minor party. Its losses are a repudiation of current President Enrique Peña Nieto’s government.

Some 30 percent of registered voters were millennials, who have no memory of the one-party state of Mr. López Obrador’s youth. They are dissatisfied with Mexico’s status quo and have voted for a change.

Let’s hope the change they get isn’t back to the future of state economic control and peso devaluation.

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