U.S. President Donald Trump will only hurt the American economy by scrapping NAFTA, according to one of his most vocal critics.

“I hope Señor Trump withdraws his insistence that he will take the United States out of the NAFTA agreement,” former Mexican President Vicente Fox told BNN on Wednesday. “That will provoke a strong impact to the economy. In this case the U.S. economy will be the great loser.”

“When we developed NAFTA, the agreement was that we would have a very strong North American region, that we would be working together to lead the world, and that we would be the largest, most competitive market in the world, so that we could meet the challenge of the east.”

Fox has been a vocal opponent of Trump’s since he took office, often using his Twitter account to lambaste the U.S. President’s policies on immigration, nuclear war, trade and especially, his proposed Mexican border wall.

Fox attacked Trump’s approach to trade, telling BNN the President’s NAFTA goals demonstrate “a lack of understanding.”

“This disruptor came into the picture with wrong ideas, with a lack of understanding [of] how [the] economy works, how trade works. He thinks it’s zero-sum. He thinks when one [country] wins, the other trader loses,” Fox told BNN.

“That’s absolutely wrong. He’s absolutely mistaken. He again thinks that if you create a job in the United States, you lose it in Mexico. Absolutely false. Trade is very positive, you create jobs in both sides, you create wealth on both sides. [It’s] surprising that a trading nation like the United States is  forgetting about these basic principles.”

However, Fox – who served as Mexican President between 2000 and 2006 – did admit that the trade pact could use updating.

“NAFTA needs some sort of a renewal,” he told BNN. “We developed that with the [George W.] Bush administration, a NAFTA-plus concept. We developed a further partnership between for security and for trading between the three of us. So, if the United States stays in, we can do things much better than what we have today.”

To do that, Fox insisted, Canada and Mexico need to stay strong and united.

“As long as Canada and Mexico stay together hand-[in]-hand we will domesticate the beast.”