WASHINGTON, May 15 (Reuters) - Reopening the North American Free Trade Agreement to address concerns raised by U.S. Democratic presidential candidates could kill the three-way pact, former Mexican President Carlos Salinas said on Thursday.

"We need to change the perception that it would be good to renegotiate the agreement. Any reopened negotiation would lead to its cancellation," Salinas, one of the architects of the pact, said in a speech to the U.S.-Mexico Chamber of Commerce.

Salinas, who was Mexico's president from 1988 to 1994, said reopening the pact would unleash "a Pandora's box" of special interests all wanting the deal changed in their favor.

He said the 14-year-old agreement had greatly benefited all three countries by eliminating costly tariffs and encouraging more cross-border investment.

Both Sen. Barack Obama and Sen. Hillary Clinton said they would want to add stronger labor and environmental provisions to the agreement, as well as change investment provisions that critics have long said favor corporate interests.

"I agree with the president that it will be a Pandora's box because we also will have issues to put on the table," Guy Saint-Jacques, deputy head of mission at Canada's Embassy to the United States, told Reuters at the event.

"I would just like to remind (you) that we are the main supplier of energy to the U.S., over $100 billion per year," Saint-Jacques said. "Canada supplies 18 percent of the oil to the U.S. We supply uranium. We supply electricity. We supply natural gas."

Under NAFTA, "Canada has to basically share its resources with the U.S. This was very contested at the time it was negotiated, but it has turned out to be a big advantage in terms of energy security for the U.S.," Saint-Jacques said.

Trade with Canada also supports some 7 million jobs in the United States, he said.

Both Salinas and Saint-Jacques said there were mechanisms within NAFTA to address any concerns the three countries have without formally reopening the pact.