This 'product by product' look at NAFTA reveals the industries in Trump's sights

In an effort to fulfill another campaign promise, the Trump administration moved to renegotiate a major trade agreement that candidate Donald Trump blamed for "massive" U.S. job losses since it was instituted more than two decades ago.

But the tone of the letter triggering a review of the North American Free Trade Agreement, sent to U.S. lawmakers Thursday, was more measured than the president's heated campaign rhetoric.

"The way we're going to go about it is product by product, country by country," Commerce Secretary Wilbur Ross told CNBC Thursday, referring to trade policy generally.

Trump's newly confirmed trade representative, Robert Lighthizer, told reporters on a conference call that NAFTA has been successful for U.S. agriculture, investment services and the energy sector, but not for manufacturing.

Certain U.S. products currently running the biggest deficits could become targets for Trump administration negotiators.

On top of that list is motor vehicles, which trade data show both Mexico and Canada running surpluses with the United States. Those data, however, mask a complex flow of parts and components back and forth across both U.S. borders as cars and trucks are assembled before rolling off assembly lines.

Another major product category, petroleum and petroleum products, could also pose problems if the goal is to eliminate all trade deficits. While the U.S. ships more oil and refined products to Mexico than it imports, American refiners are heavy consumers of Canadian oil. Trump has pledged to dramatically increase U.S. oil and gas production, but Canadian supplies will likely remain an important American energy source for some time.

'Disaster'

Trump has frequently called the 23-year-old trade pact a "disaster" that has emptied U.S. factories and sent well-paid manufacturing jobs to Mexico.

Back in March, a draft of a letter sent to lawmakers said the Trump administration would seek tax equality and the ability to slap tariffs on Mexican and Canadian imports if they pose a serious injury threat to U.S. industry.

But in the final letter sent Thursday, Lighthizer called on "Congress and American stakeholders to create an agreement that advances the interests of America's workers, farmers, ranchers and businesses."

90-day review

The letter triggers a 90-day review process that's required before talks with Mexico and Canada begin over specific changes in the three-country trade deal.

Lighthizer told reporters he hopes to complete negotiations by the end of 2017, a further indication that the Trump administration's focus has shifted to updating and improving the agreement rather than scrapping it and starting over.

"As a starting point for negotiations, we should build on what has worked in NAFTA and change and improve what has not," Lighthizer said. "If renegotiations result in a fairer deal for American workers there is value in making the transition to a modernized NAFTA as seamless as possible."