Other independent candidates will also appear on the ballot but have no real chance of victory. All of them, however, beyond their political differences and except for Mr. López Obrador, share a respect for democracy.

Mr. López Obrador has promised “a change of regime.” Mexican voters should carefully consider what he means, given the precedents.

To begin with, he has said that he does not believe in the existence of Mexican democracy, though it has been in the context of its rules, institutions and freedoms that he has gained the likelihood of power. Nor does he trust in the National Electoral Institute. After losing the 2006 election by a wafer-thin margin (0.58 percent), he declared the election fraudulent and led his followers in occupying the Paseo de la Reforma, the central artery of Mexico City, an action rejected by the public. Defeated by a larger margin (6.63 percent) in 2012, he again claimed fraud. He has continued to show disdain for the institutions of liberal democracy. “To hell with their institutions,” he famously said in 2006 and has not disavowed his assertion. And he recently accused the Supreme Court of being an instrument of the oligarchy used to dominate the people.

There is a genuine linkage of religious fervor (which it seems just to call messianic) between Mr. López Obrador and his followers. Confident in that connection, he has shown an unbending intolerance toward criticism from the media and intellectuals. He has a disqualifying adjective for every group that doubts or opposes him: “fakes,” “conservatives,” “sellouts.” He has called the press “fifí” (bourgeois). He has proved to be incapable of self-criticism and shows a significant tendency to divide the country between “the people” who support him and all the others, who support “the mafia in power.”

Mr. López Obrador trusts so much in the sheer force of his charisma that he has promised to “bring Trump to his senses.” He wants to bring peace to Mexico by exploring the possibility of granting amnesty to drug traffickers and producers. “Only I can fix corruption,” he has stated, and he recently announced that he will call for the drafting of a “moral Constitution” that can inaugurate a “loving republic.”

Mr. López Obrador has surrounded himself with politicians and union leaders formerly of the P.R.I. who were directly involved in gross acts of corruption. He shows a real concern for alleviating poverty though is as yet insufficiently specific in his proposals. Many liberal Mexicans fear that he will reverse the opening to private and foreign investment in Mexican oil production and will choose to protect the domestic economy from international competition.

My own major concern, however, is his attitude toward our still fragile democracy. In his defense, his supporters point to his record as mayor of Mexico City (2000-05), but that position did not remotely involve the power that would accrue to him as president. Were Mr. López Obrador to choose to incite popular mobilizations and plebiscites, his government could call for a Constituent Assembly, and move toward annulling the division of powers and subordinating the Supreme Court and other autonomous institutions after restricting the freedom of the media and silencing any dissenting voices. In such circumstances, Mexico could once again become a monarchy, though messianic and in the style of a caudillo without republican costuming: “the country of a single man.”

It is to be hoped that the legitimate discontent of Mexicans and the urgent need for change does not lead to the demise of our fledgling but genuine democracy.