OTTAWA — In a cross-border political attack, John McCain said yesterday that Barack Obama's opposition to the North American Free Trade Agreement is "nothing more than retreating behind protectionist walls."

The Republican presidential nominee-in-waiting added that if he wins the White House, "have no doubt that America will honor its international commitments — and we will expect the same of others."

McCain did not mention Obama by name as he spoke before the Economic Club of Canada, a business organization whose membership cheered his remarks.

Obama, on the campaign trail in Florida, shot back: "What's interesting to me is that he chose to talk about trade in Canada instead of in Ohio or Michigan. ... I think Senator McCain should have shared some of his views there to American voters."

McCain's trip to Canada was unusual if not unprecedented for a presidential candidate, one that his campaign paid for yet aides insisted was not political.

Democrats criticized plans for a scheduled $100-per-person "finance event" and raised questions about U.S. Ambassador David Wilkins' involvement in the trip. McCain's aides said Wilkins had done nothing wrong. They also countered that the money was to pay the cost of the Economic Club luncheon — then amended that to say merely that none of the money went to the campaign.

The free trade agreement is intensely controversial in the United States — supported by most businesses, opposed by many unions — and has already emerged as a flashpoint in the presidential race.

McCain supports it, while Obama and former rival Hillary Rodham Clinton vied for support among blue-collar workers in the Democratic primaries by stressing their desire to force changes.

"Since NAFTA was concluded, it has contributed to strong job growth and flourishing trade. Since the agreement was signed, the United States has added 25 million jobs and Canada more than 4 million," McCain said.

In an unmistakable reference to Obama, he added, "Demanding unilateral changes and threatening to abrogate an agreement that has increased trade and prosperity is nothing more than retreating behind protectionist walls."

Aides said that was a reference in part to comments the Illinois senator had made in a Feb. 26 debate during the primaries.

"I will make sure that we renegotiate in the same way that Senator Clinton talked about," Obama said at the time. "I think we should use the hammer of a potential opt-out as leverage to ensure that we actually get labor and environmental standards that are enforced."