Mexican President Enrique Peña Nieto, Canadian Prime Minister Justin Trudeau and President at the National Gallery of Canada at the start of the North American Leaders’ Summit in Ottawa on Wednesday. (Kevin Lamarque/Reuters)

By Mack McLarty and Nelson Cunningham,

Mack McLarty, chief of staff to President Bill Clinton from 1993 to 1994, is chairman of McLarty Associates, a global business consulting firm. Nelson Cunningham was a special adviser to President Clinton on Western Hemisphere affairs and is president of McLarty Associates.

North America the continent has been here for tens of millions of years. North America as a political idea never quite arrived. But North America the economic powerhouse has reigned supreme for nearly a century, becoming the largest and strongest in the world, an industrial dynamo, a commodities cornucopia and a magnet for millions upon millions of immigrants seeking a better life.

But we may be about to witness the end of the supremacy of that North American economy.

North America has prospered because Canada, the United States and Mexico are tightly integrated markets. Factories in Mexico produce goods for its neighbors to the north. Factories near the border of Canada and the United States are so dependent on each other that the parts of a North American automobile have crossed the border an average of seven times before it rolls off the final assembly line.

North America has also prospered because its populations have mixed and mingled. Those living on each side of our northern border have similar polyglot immigrant backgrounds, the same drive for success, the same frontier spirit that developed their great collective heartland. Those living on each side of our southern border share rich cultures based on faith, family and hard work.

On Wednesday, the leaders of Canada, Mexico and the United States, representing a combined 475 million people, came together in Ottawa to discuss climate change and other vital issues. But they needed to — and did — set their sights higher. For we North Americans need bold action from our leaders to fight back against a clear and present danger to our well-being: the dissolution of our great North American economy. Last week’s Brexit vote lends a fresh urgency to this task.

What is this threat? It is the resurgence in the United States of protectionism and isolationism, nativism and xenophobia. We see elements of each of these across the political spectrum. They find their strongest embodiment, however, in the candidacy of Donald Trump. His meteoric rise to become the presumptive Republican nominee for president is premised in large part on his promise to dismantle the cooperation across borders that is the foundation of our economic security.

Trump’s policies would have a radical impact on our neighbors. He promises to impose a tax of 20 percent, 35 percent or more on goods imported from Mexico, Canada and other countries. He would dissolve the North American Free Trade Agreement and restore hundreds of protectionist measures. He would ban immigration and visits from a million Canadian adherents of Islam, its second-largest religion. He would redo NATO, with untold implications for its unified North American defense systems linking U.S. and Canadian armed forces. And, most famously, he has vowed to build a wall on the Mexican border (and to force Mexico to pay for it).

If enacted, these policies themselves would slow the U.S. economy. And they would surely provoke reactions from our neighbors, exacerbating the damage. Mexico has its own nativist and protectionist traditions. A Canada barred from our markets would have incentive to retaliate — and look elsewhere, principally to China, its second-largest trading partner.

There is no doubt that NAFTA and foreign trade have long been controversial in the United States. Sen. Bernie Sanders’s campaign was fueled by protectionist concerns as much as Trump’s. Support for free trade has waned across the board, with Republican congressional leaders resisting President Obama’s 12-nation Trans-Pacific Partnership agreement, and Hillary Clinton expressing her own reservations.

And yet, by many measures the North American economies have never been stronger. Taken together, they are substantially larger than those of China or the European Union. Since 2010, the United States has added 1 million manufacturing jobs. Meanwhile, exports to Canada support more than 1.5 million jobs in the United States. In Mexico, the manufacturing sector is forecast to grow nearly 4 percent per year over the next 18 years.

The North American industrial renaissance — benefiting from cheap and abundant energy and commodities, competitive wages for highly skilled workers, and the largest internal market in the world — has been rightly celebrated. Many leading American companies are championing the trend toward “reshoring” and “near-shoring” — bringing production back to the NAFTA countries from Asia and other markets.

Meanwhile, the North American populations reside in a unique demographic sweet spot: They are old enough to have savings but young enough to have energetic workers and consumers. Compared with aging populations in China, Japan and Europe, or the too-young populations of the less developed world, North America has just the right balance. Why? Because of our open and flexible immigration policies.

President Obama, Mexican President Enrique Peña Nieto and Canadian Prime Minister Justin Trudeau had a formal agenda in Ottawa, working to confront the challenges of securing our borders, harmonizing national regulations and coordinating action on climate change. But standing together in front of the media, they rose to the further challenge posed by protectionism and nativism. They made clear that vows to destroy the North American economic alliance feed dangerous illusions that will leave each country weaker in the face of fierce global competition. In the months ahead, we will need more such leadership. We share this continent, and our common future is at stake.