An ABC News report identified Harper's chief of staff, Ian Brodie, as the source of the first leak of meeting details to reporters. Harper has said Brodie had nothing to do with it.
Liberal foreign affairs critic Bob Rae said on his blog Tuesday that it was clear the Tories are trying to help Republican friends at the expense of Canadian interests.
The controversy could play to the Republicans' advantage in November in Ohio, a potential swing state where job losses have made the 15-year-old free trade deal highly unpopular.
Obama and Democratic competitor Senator Hillary Clinton have both said they want to reopen the free trade deal between Canada, the U.S. and Mexico to ensure better environmental and labour standards.
International Trade Minister David Emerson and Finance Minister Jim Flaherty said U.S. officials should not forget the benefits of the agreement and hinted Canada could respond to a NAFTA pullout by renegotiating U.S. access to Canada's oil.
Harper says information leak unfair to Obama, probe in progress