Barack Obama has ratcheted up his attacks on NAFTA, but a senior member of his campaign team told a Canadian official not to take his criticisms seriously, CTV News has learned.

Both Obama and Hillary Clinton have been critical of the long-standing North American Free Trade Agreement over the course of the Democratic primaries, saying that the deal has cost U.S. workers' jobs.

Within the last month, a top staff member for Obama's campaign telephoned Michael Wilson, Canada's ambassador to the United States, and warned him that Obama would speak out against NAFTA, according to Canadian sources.

The staff member reassured Wilson that the criticisms would only be campaign rhetoric, and should not be taken at face value.

But Tuesday night in Ohio, where NAFTA is blamed for massive job losses, Obama said he would tell Canada and Mexico "that we will opt out unless we renegotiate the core labour and environmental standards."