The New York senator contends Obama's campaign sent a memo to a Canadian diplomat in Chicago reassuring the northern neighbors his railing against the North American Free Trade Agreement was "more about political posturing than a clear articulation of policy plans" and urging Canada not to take his statements "out of context."

NAFTA is a hot topic in Ohio, where global free trade is widely considered an unfair trade-off of U.S. manufacturing jobs for cheap imports. Ohio Democrats vote Tuesday in a primary Clinton likely must win to maintain a realistic chance of becoming the Democratic nominee.

Clinton labeled the memo "the old wink-wink" between the Illinois senator and Canada, one of the three signatories to NAFTA. Mexico and the United States are the others.

"I don't think people should come to Ohio and give speeches that are very critical of NAFTA and then … we find out that your chief economic adviser has gone to a foreign government and basically done the old wink-wink, don't pay any attention, this is just political rhetoric," she told reporters aboard her campaign plane.