WASHINGTON -- Donald Trump has delivered his most explicit threat to smash the North American Free Trade Agreement just as the continent's three leaders gather to discuss building new ties.

If elected president, the presumptive Republican nominee said Tuesday he would inform Mexico and Canada of his desire to immediately renegotiate a better deal for the U.S.

He said their refusal would prompt him to invoke the agreement's Article 2205, which allows a party to withdraw on six months' notice.

"They're so used to having their own way," Trump said of America's neighbours.

"Not with Trump. They won't have their own way ... NAFTA was the worst trade deal in the history -- it's, like, the history -- of this country."

The warning came in an anti-globalization speech that solidified Trump's position as the most protectionist Republican presidential candidate in generations.

He listed a series of complaints about American trade policy as he read a pre-written address titled, "Declaring American Economic Independence."

It occurred on the eve of Wednesday's Three Amigos summit in Ottawa where the NAFTA leaders plan to announce closer co-operation in several areas, against the headwinds of economic nationalism now blowing on different continents.

For Trump, that growing resistance to global integration is cause to celebrate.

He saluted Britain's vote to leave the European Union. He even used a turn of phrase similar to one from anti-EU leader Nigel Farage, who taunted European colleagues Tuesday for mocking his Brexit vision: "You're not laughing now, are you?"

In a similar sneer at the forces of global integration, Trump said he'd spent years complaining about trade with China: "Nobody listened. But they're listening now."

Trump's two main complaints were about events shaped by the presidency of Bill Clinton, the spouse of his likely general-election opponent: NAFTA in 1993, and the lengthy negotiations that led to China entering the World Trade Organization in 2001.

He proposed seven remedies.

One is to withdraw from the 12-country Trans-Pacific Partnership plan. Another is to label China a currency manipulator and impose punitive tariffs on its products.

Redoing or rescinding NAFTA is another.

In his speech, Trump never mentioned Canada specifically. Even after the speech, his campaign issued talking points complaining about the trade practices of Mexico -- but not of Canada.

Yet one trade lawyer in Toronto said the policies he espouses have clear implications for the northern neighbour; three-quarters of Canadian exports go to the U.S.

"He is clearly the most protectionist presidential candidate, probably, since the 1920s," Mark Warner said.

"Presidential powers are limited but a president with this kind of a trade agenda could easily make life difficult for trading partners by pushing presidential executive powers to the limit. The worst case for Canada is that he might push (Hillary) Clinton to be more protectionist in border states if the contest narrows."

Nearly two dozen consecutive polls show him losing the popular vote to Clinton.

His comeback hopes appear to hinge on the anti-trade, anti-elite, anti-foreign sentiment of the likes that propelled Farage's forces to a stunning win.

Trump has repeatedly stated his desire to redraw the electoral map.

As a case in point, he delivered his trade speech Tuesday in Pennsylvania -- a state that hasn't voted Republican in decades. He'd need that state and other northern, former industrial powerhouses like Michigan and Ohio, if he loses states with growing Latino populations like Florida, Virginia and Colorado.

Even if he's elected, Warner said, a protectionist president would find himself in a complex fight.

He said Trump could seek to leave NAFTA, but would need Congress to snap tariffs back into place. Such a move would almost guarantee a lawsuit over the extent of presidential power, he said.

Warner said presidents do have the power to implement emergency actions without Congress, and can get the Department of Commerce and International Trade to ramp up trade-remedy cases.

Trump's domestic opponents were saying far worse things about his speech.

They called it economically illiterate and hypocritical.

After all, they pointed to reports of Trump-branded clothes being made in Mexico, ties made in China, shirts made in Bangladesh, furniture made in Turkey and Germany, picture frames from India, and bar products from Slovenia.

This self-styled defender of workers has also testified in a court case about using illegal labour in the project to build the skyscraper that now houses his campaign headquarters.

Polish workers who were to be paid a few dollars an hour for demolition work in the 1980s complained about their supervisors withholding payment on the Trump Tower project.

Now the Trump of 2016 is campaigning against the wealthy international trade-championing elite.

"Globalization has made the financial elite who donate to politicians very wealthy. But it has left millions of our workers with nothing but poverty and heartache," he said.

"The era of economic surrender will finally be over ... America will be independent once more."

His opponents might not find themselves on the firmest ground if they question his consistency. When they first ran for president, both Hillary Clinton and Barack Obama endorsed a renegotiate-or-rescind approach to NAFTA.

On Tuesday, the White House was lauding the old agreement.

Obama spokesman Josh Earnest did so while warning against making simple comparisons between the situation in North America and Europe -- given that continent's far more elaborate integration featuring open borders, free movement of labour, and a multinational currency.

"Countries in North America have pursued a different strategy, and one that has worked well for us," he said.

"It is a strategy that has enhanced the economies of all of our countries, it's enhanced the national security of all of our countries and it certainly has made North America the most successful continent in the world."