Canada's CTV reports:

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."

Late Wednesday, a spokesperson for the Obama campaign said the staff member's warning to Wilson sounded implausible, but did not deny that contact had been made.

"Senator Obama does not make promises he doesn't intend to keep," the spokesperson said.

Low-level sources also suggested the Clinton campaign may have given a similar warning to Ottawa, but a Clinton spokesperson flatly denied the claim.

Late Update: Here's the video of the report.