The presidential candidates traded accusations of flip-flopping Friday, with John McCain's campaign saying Barack Obama was waffling on trade and Obama charging his rival had done a turnabout on energy policy.

The dispute played out through a series of conference calls, memos, public addresses and press conferences, as the candidates campaigned in different countries.

Both presidential candidates are trying to call each other out for reversing course -- but McCain's campaign was working extra hard to make the flip-flopper label stick. That tactic was used by Republicans to defeat Democratic nominee John Kerry in 2004. McCain also used that label against his former GOP rival Mitt Romney during this year's primary race.

McCain took his shots Friday from Canada, speaking in Ottawa to the Economic Club of Canada. There he chided Obama for saying he would renegotiate the North American Free Trade Agreement.

Without mentioning Obama by name, McCain said that threatening to back out of the deal is "nothing more than retreating behind protectionist walls."

McCain emphatically supports the 1994 trade deal, while Obama said during his primary campaign that he would renegotiate the deal and use the threat of opting out as "leverage."

"Since NAFTA was concluded ... we have established North America as the world's largest economic market and the integration of our economies has led to greater competitiveness of American and Canadian businesses. I'm proud of what we've accomplished together," McCain said. "And even now, for all the successes of NAFTA, we have to defend it without equivocation in political debate."

The Arizona senator 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."

But while McCain criticized Obama for doubting the trade deal, his campaign was highlighting discrepancies in Obama's NAFTA stance.

The free trade agreement is intensely controversial in the United States -- supported by most businesses, opposed by many unions.

Though Obama and former rival Hillary Clinton stressed their opposition to NAFTA during the primary, Obama seemed to soften his tone during a recent interview on trade with Fortune magazine. He told the magazine that, "Sometimes during campaigns the rhetoric gets overheated and amplified," and that he doesn't want to do anything "unilaterally" with the trade agreement.

During the primary, Obama also weathered reports that his economic adviser had met with Canadian officials to assure them that his tough talk on NAFTA was just posturing.

Obama said Friday that his position is essentially "unchanged" and that the full Fortune magazine interview reflects that.

"I believe in free trade ... but that doesn't preclude us from making sure that we structure our trade agreements in such a way that doesn't have the United States taken advantage of," he said.

But McCain's campaign released a statement criticizing Obama and accusing him of changing his tune.

"For months, Barack Obama said that he would 'make sure that we renegotiate' NAFTA, demanded unilateral changes and threatened to unilaterally withdraw if he did not get his way. Barack Obama knew better. America has not had a protectionist president since Herbert Hoover, but Barack Obama held his position anyway to further his cynical political purposes in the primary campaign," the statement said.

"Now he claims: 'I'm not a big believer in doing things unilaterally.' Barack Obama should know words matter -- especially in a campaign based on rhetoric rather than a record of accomplishment."

The campaign has increasingly tried to portray Obama as all talk after he appeared to back out of holding a series of joint town hall meetings with McCain and then reversed positions Thursday to decline public financing for his general election campaign.

McCain adviser Steve Schmidt released a memo, called "Words Matter," highlighting those positions Friday.

Meanwhile, Obama criticized McCain for reversing his position on offshore oil drilling, shortly after arriving in Florida for a fundraiser.

In McCain's 2000 campaign, he said he favored the federal government's 27-year moratorium on offshore drilling. This week, he said he supports lifting it to give states the option to drill.

Pointing out that McCain once had a different position, Obama said McCain probably thought his new position would poll better.

But Obama said opening up the U.S. coastline to oil exploration would not give Americans any appreciable savings until 2030, and that he would keep in place the federal ban.

"McCain's proposal, Bush's proposal to drill offshore would not provide any relief this year, next year, five years from now," Obama said. "If I thought there was any evidence at all that it would save people money I would consider it, but it won't. And Senator McCain knows that."

Obama also criticized his Republican rival over his energy policies at a roundtable discussion with 16 Democratic governors in Chicago. There he said the Arizona senator's proposal to allow offshore oil drilling "makes absolutely no sense at all."

Obama told the governors he couldn't wait to hear what their Republican colleague Arnold Schwarzenegger of California would have to say about McCain's plan. Told that he'd already spoken out, Obama said, "He's thumbs down, right?"

He is. Schwarzenegger said Wednesday that California's coastline is "an international treasure" that must be protected.

McCain has made several trips outside the United States since he became a presidential contender, including European and Middle Eastern countries.

His trip to Canada was unusual. While he arrived aboard his chartered campaign jet, he was greeted on the tarmac by Ambassador David Wilkins. He said it was not a political journey, yet told reporters he did not feel it was appropriate to have U.S. taxpayers pick up the cost.

He later downplayed the significance of his visit, saying, "This is not any different from any travel I have conducted."

In his speech, McCain expressed his appreciation for Canada's deployment of 2,500 troops to Afghanistan, and skipped lightly over Iraq, where the government declined to send forces.

Aides said in advance McCain would come to Canada to highlight trade, and there has been widespread speculation that he will soon travel to Mexico and perhaps elsewhere to make the same point.

FOX News' Bonney Kapp and Mosheh Oinounou and The Associated Press contributed to this report.