“You didn’t have to have a PhD in economics to figure that one out,” Sanders said, asserting that NAFTA was written by corporate interests with the aim of driving down labor costs with no regard to the effects on American workers.

“This is a criticism of Secretary Clinton’s trade policies, which have been a disaster,” Sanders said of the former secretary of state, senator from New York and first lady. He was flanked by several supportive labor union members.

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Sanders acknowledged that Clinton now opposes the Trans-Pacific Partnership, a proposed agreement being championed by President Obama, but suggested that she reached that position only because she is running against him for the Democratic nomination.

“She voiced a lot of support over the years for the concept of the TPP,” Sanders said.

“She was very reluctant to come out in opposition,” he said. “She’s running for president. She concluded it was a good idea to come out against the TPP.”

Clinton has won more labor union endorsements than Sanders. When asked how her trade policies could be that bad given that support, he said reporters should ask the labor unions. He noted that several national unions have endorsed him and said that tens of thousands of rank-and-file members of other unions also are supportive.

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“Sometimes the leadership does not necessarily speak for the membership,” he said.

Sanders’s criticism of Clinton comes ahead of a key primary next week in which he will try to prove that he has the staying power to compete with Clinton, who has opened a significant lead in the delegate count for the nomination.

In a conference call with reporters later Thursday, Clinton policy adviser Jake Sullivan questioned Sanders's commitment to rebuilding the manufacturing sector, noting that he hasn't put forward a manufacturing plan during the campaign.

"In his years in Congress, he has been AWOL when it comes to putting forward proposals that would help lift up American manufacturers," Sullivan said.

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He also cited Clinton's efforts as a New York senator to fight the Chinese government for imposing "discriminatory tariffs" affecting companies in her state and for pushing to help out the auto industry when it was in crisis.

Sanders’s advisers argue that Michigan is fertile ground for the self-described democratic socialist because of the number of trade-related job losses that state has faced. Sanders debuted a television ad on Wednesday that highlights his views on trade.