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."

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.

During Tuesday's debate, she said that as president she would opt out of NAFTA "unless we renegotiate it."

Canadian Finance Minister Jim Flaherty said Wednesday that the candidates' criticisms of NAFTA were misguided.

"(They) should recognize that NAFTA benefits the U.S. tremendously," he said. "Those who speak of it as helpful to (just the) Canadian or Mexican economies are missing the point."

Liberal MP and finance critic John McCallum told Canada AM that the U.S. pulling out of NAFTA "would be a disaster for Canada."

But he added, "I hope and I believe that it's politics, because they're in a high-stakes contest. I believe after this nominee is decided, this issue will go away."

John Fortier, a senior fellow at the American Enterprise institute, said that in an effort to gain votes in the anti-NAFTA state of Ohio, each candidate might find themselves "locked-in" to their pledge to renegotiate NAFTA.

"Last night, both candidates really locked themselves in to at least doing some serious renegotiation," Fortier told Canada AM. "But how serious they are and what the changes (will be) . . . that's another question.

"But I don't know how Barack Obama or Hillary Clinton can get out of last night's very clear pledge that they are going to use the opt-out (clause) as a threat to do some serious renegotiation."

Crucial primaries in Ohio and Texas are just one week away.

During Tuesday night's debate, each candidate was quite specific about using the six-month opt-out clause in NAFTA, to pressure Canada and Mexico into renegotiating the deal.

The March 4 primaries are seen as vital for each candidate, but particularly Clinton. It's expected that without a decisive win in both Texas and Ohio, she has no chance of winning the Democratic nomination.

Clinton once had a large lead in each state, but recent polls are showing the candidates as close to even, with Obama surging ahead.

Early polls show that there is a strong possibility of a Democrat in the White House in January 2009.

Obama, in particular, is surging in popularity throughout the U.S. and some polls give the Illinois senator an almost double-digit lead if he were to run head-to-head against the expected Republican candidate, John McCain.

With a report by CTV's Tom Clark in Washington