By Jason Lange

MEXICO CITY, March 6 (Reuters) - Mexican factory owners are shrugging off threats by U.S. presidential candidates to retool a major free trade deal but say they would worry if the talk continues after election day in November.

As they vie for the Democratic Party nomination, Barack Obama and Hillary Clinton have vowed to tell Canada and Mexico that the United States will pull out of the North America Free Trade Agreement unless labor and environmental standards are renegotiated.

Yet even if one of them wins the November election against presumptive Republican nominee John McCain, a NAFTA supporter, few see Washington easily reopening the trade deal, the fruit of years of thorny negotiations.

"I really doubt they would go through with it. They are just playing politics," said Cesar Castro, who heads Mexico's association of maquiladoras, the assembly-for-export factories that dot the border with the United States.

Free trade has become a contentious issue in the race to become the Democratic Party's nominee, with Obama and Clinton both jockeying to woo blue-collar voters who feel removing barriers on imports has cost American jobs.

Mexican businessmen and investors say NAFTA has brought clear benefits to their American partners by making their companies more competitive and profitable.

Castro said killing NAFTA or changing it to favor American workers would shut down Mexican factories and hurt employment in Mexico.

"Illegal migration would spike and we all know they don't want that," he said.

By reducing or eliminating tariffs, NAFTA has helped make Mexico the No. 3 business partner with the United States, after Canada and China.

"There are net benefits on both sides of the border, so I don't think they will touch it," said Jose Antonio Abogaber, who heads the chamber of shoe makers in the industrial state of Guanajuato.

Conservative Mexican President Felipe Calderon's government has made clear it is happy with NAFTA the way it is, while warning against any move away from free trade.

"NAFTA has been successful for each of the three countries," Economy Minister Eduardo Sojo told reporters in Toronto earlier this week. "What we need is more integration, not less integration."

(Editing by Eric Beech)

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