In a rare foray into politics, Slim -- the world's fifth-richest man, according to Forbes magazine -- hit back at the fiery leftist leading the race for the July 1 vote, Andres Manuel Lopez Obrador, without mentioning him by name.
"Canceling the project would amount to canceling the economic growth of the country," said Slim, whose construction company CICSA was awarded the $4.7 billion contract to build the airport's terminal in consortium with six other companies.
Lopez Obrador, a former Mexico City mayor, has threatened to halt construction on the airport, calling it a waste of taxpayer money and alleging the project was marred by corruption.
He has instead proposed repurposing a military airbase to alleviate the saturation of the capital's current international airport, which handled nearly 45 million passengers last year.
Slim said that would only be a "patch," not a long-term solution to the pressure on one of Latin America's busiest airports.
He also defended the new airport as a boon for residents of the area northeast of the city where it is being built.
"It is a magical transformation of this area of the city," he told a press conference at his offices.
The new airport will create up to 450,000 jobs and have the capacity to handle 125 million passengers a year when fully operational, according to the Mexican government.
Slim made most of his estimated $71.7 billion fortune in telecoms, but he is deeply involved in the project.
His son-in-law, Fernando Romero, designed the sleek new terminal building together with British architect Norman Foster.
Lopez Obrador wasted no time firing back.
"This airport they want to build is going to cost the country a fortune," he told journalists after a campaign rally in the northern state of Sonora.
"It's a bottomless pit. It's not a good deal for the nation."
Lopez Obrador has also caused investor jitters by vowing to review contracts with private companies that bought stakes in Mexico's oil sector in a landmark privatization under President Enrique Pena Nieto.
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