Justin Trudeau's critics have a point when they claim he is Stephen Harper's fourth term, dressed up in a smile. What they don't understand is how lucky we are this is true.

If, on the really big stuff – the stuff that shapes the direction of a country – the centre left and the centre right agree, the political culture is healthy. Canada's political culture is healthy. America's isn't.

It took real courage for Mr. Trudeau to approve the expansion of the Trans Mountain pipeline from Edmonton to Burnaby, B.C. Members of his own caucus opposed the decision. Vancouver Mayor Gregor Robertson denounced it. Environmental and indigenous groups are vowing to fight the pipeline to the end.

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But dollars to doughnuts, most Canadians support Mr. Trudeau's decision. Canada thrives or suffers based on the health of its exports. That's why the Harper government pushed hard for Pacific pipelines and that's why the Trudeau government approved Trans Mountain – while vetoing the environmentally riskier Northern Gateway.

That's why the Harper government launched free-trade negotiations with Europe and the Trudeau government successfully concluded them.

That's why the Conservatives helped negotiate and the Liberals tacitly support the Trans-Pacific Partnership trade agreement.

But the TPP is dead. President Barack Obama strongly backed the deal, but president-elect Donald Trump has said he will veto it.

Mr. Trump is determined to dismantle all of the major achievements of the Obama presidency. Tom Price, whom the president-elect nominated for health secretary Tuesday, is an implacable foe of Obamacare. Despite Mr. Trump's promise last week to keep an "open mind" on climate change, his chief of staff, Reince Priebus, told Fox News Sunday that the administration's "default position" is that the scientific research around global warming is mostly "a bunch of bunk." Mr. Obama opened the door to normalizing relations with Cuba; Mr. Trump vows to slam it shut.

A new government that dedicates itself to reversing the policies of the previous government is the product of a deeply polarized electorate, which can lead to policy paralysis, confusion in foreign policy, erosion of trust in democratic institutions and, ultimately, violence.

Consider another big issue: immigration. The Diefenbaker government dismantled anti-Asian policies. The Pearson government created the points system that allows anyone from anywhere to enter Canada if they qualify. The Mulroney government opened the door to 250,000 immigrants a year. The Chrétien government kept that door open; the Harper government sustained and ultimately increased the intake; and the Trudeau government is moving the annual target to 300,000 a year.

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In contrast, Democrats and Republicans have argued for decades about illegal immigrants, with the left advocating amnesty and the right opposing it. Mr. Obama sought to protect illegal immigrants who have been living and working in the United States for many years; Mr. Trump vows to impose mass deportations.

Democrats and Republicans violently disagree on taxes and deficits. The Mulroney government created the instruments that made it possible for the Chrétien government to eliminate the deficit, giving the Harper government the fiscal room to combat the 2008-09 recession. The Conservatives then brought the budget back into balance, making it possible for the Liberals to invest heavily in infrastructure.

Both Liberals and Conservatives support strict banking regulation; the Trump administration wants to dismantle the controls the Obama administration imposed on Wall Street. Both Liberals and Conservatives support sending troops to protect Latvia. Mr. Trump intends to cancel Mr. Obama's agreement on nuclear weapons with Iran.

There are lots of things that Grits and Tories fight over: How best to combat climate change; how best to fund child care; the right retirement age; the right sentences for criminals; whether pot should be legal; which fighter jet to buy. But these are arguments on the margins.

The Americans fight tooth and nail over everything. They have gone from electing Barack Obama to electing Donald Trump. And they are tearing themselves apart.