Open this photo in gallery A closed Department of Homeland Security building and U.S. Citizenship and Immigration Services field office is shown in this file photo taken in Tukwila, Wash., on March 3, 2020. JASON REDMOND/AFP/Getty Images

The tweet may simply have been a bit of raw meat for his base. But if Donald Trump really does plan to ban all immigration into the United States, that would be the worst act of his presidency, which is saying something.

Banning immigrants would amplify one of the most important demographic trends of our time: declining fertility rates among millennials and Gen Z. Babies who are not being born must be replaced with people brought in from abroad. The inevitable alternative is increased joblessness and economic decline. This is as true for Canada as it is for the United States.

The U.S. President tweeted Monday night that “in light of the attack from the Invisible Enemy, as well as the need to protect the jobs of our GREAT American Citizens, I will be signing an Executive Order to temporarily suspend immigration into the United States!”

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On one level, the tweet in nonsensical. Most countries have closed their borders as they grapple with this pandemic. Until the novel coronavirus is brought under control one way or another, both Canada and the United States will be largely closed to immigration.

But once economic life returns to something approaching normal, then not only will immigration need to return to previous levels – currently 340,000 a year in Canada’s case; traditionally more than one million a year in the case of the United States – they should be increased to make up for the immigrants who should be arriving today but aren’t.

“We desperately need immigration,” said Benjamin Tal, deputy chief economist of the Canadian Imperial Bank of Commerce, in an interview. “We are an aging society that in 10 or 15 years will be totally dependent on immigrants that we are getting now."

That Canadian fertility rate declined from 1.6 children per woman in 2010 to 1.5 last year, while in the United States it fell to 1.7, far below the 2.1 needed to keep a population stable. Fertility rates have been below the replacement rate in most developed countries for decades, which is why immigration is required for population growth.

But in recent years, a new trend has emerged. The birth rate for millennials and Gen Z is lower than it was for Gen X and the baby boomers. A study by Ron Kneebone, a professor of economics at University of Calgary, showed that fertility levels for Canadian women under 30 declined significantly between 2000 and 2017.

Citing correlations between past economic downturns and fertility, Prof. Kneebone strongly suspects that the current economic crisis will lower Canada’s birth rate even further. “When I’m uncertain whether I’m going to keep my job and be able to pay the mortgage, is this the time to be having another kid? Probably not," he said in an interview.

And, as he points out, countries with low fertility, little immigration and declining populations struggle to preserve economic growth. Just ask Italy or Japan.

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Mr. Trump has played to nativist sentiments throughout his presidency. The fabled wall on the Mexican border, banning visitors from certain Muslim countries, suppressing immigration levels overall – these are policies designed to appeal to Americans who fear their white, Christian culture is being overwhelmed by foreigners.

In that context, Monday’s tweet should be seen not as an update on pandemic border control but as a racist reassurance to his MAGA base.

But restricting immigration would be disastrous for the United States. One-quarter of all health care workers in the U.S. are immigrants. They account for half of all the entrepreneurs whose startups grew to be worth US$1-billion or more. They account for almost 40 per cent of U.S. Nobel Prize winners.

The American health care system and the American economy depend on immigrants. By stoking anti-immigrant sentiments, Mr. Trump is threatening the future of both.

Polls show that most Canadians continue to support the high level of immigration that this country has enjoyed under both Conservative and Liberal governments for three decades. But Quebec Premier François Legault has reduced the number of immigrants coming into the province and has said he may reduce the level even further in the wake of the pandemic.

That’s a policy for economic suicide. When the borders reopen, Canada should increase its immigration target above next year’s goal of 350,000. Every immigrant we don’t bring in this year and next is an opportunity lost for Canada’s future, unless we make it up further on.

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Neither the U.S. nor Canada can afford to close its doors.

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