As Liberals and Conservatives hold caucus retreats this week and New Democrats prepare to vote for a new leader, all three national parties face challenges that will define their brands. All three confront, as well, a political wildcard that could upend everything.

Because they are the governing party, barnacles were bound to accumulate on the Liberal hull, slowing progress. The Omar Khadr settlement. Amnesty applicants flooding across the border. The promises made – electoral reform, Indigenous rights and services, restoring home delivery at Canada Post – that weren't, or at least haven't been, kept.

The biggest problem for Prime Minister Justin Trudeau, however, is his commitment to class-based politics. The Liberals consciously chose to tax the rich and pass the savings on to the middle class. But who is middle class? Just about everyone answers: I am. Doctors, farmers, small-business owners and others who have incorporated are furious over proposed changes that would increase their income taxes. "We're middle-class, too!"

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Bill Morneau: Tax changes are about levelling the playing field

For subscribers: Trudeau stands firm on tax plan, but open to 'tweaks'

Finance Minister Bill Morneau says that only people making healthy six-figure incomes will feel the pinch. But will Canadians, broadly, consider the tax hike fair? Or will the Liberals pay a price for pitting one cohort of the middle class against the other?

That said, polls show the Liberals as popular at mid-term as they were on election day. For any governing party, that is a sweet place to be.

The Conservatives have a steeper hill to climb. Andrew Scheer, the new leader, should ignore snarky columns writing him off as colourless. All opposition leaders are written off until the middle of an election campaign, when things start to get interesting.

Mr. Scheer's bigger problem lies south of the border. U.S. President Donald Trump has poisoned the conservative well. His openly racist words and deeds pit nativist white voters against everyone else. His latest threat to deport those who were brought into the United States illegally as children is positively Dickensian – Charles Dickens would have loved the name "Trump" for one of his most heartless characters – even if it does force Congress to act.

To make matters worse for Mr. Scheer, there are elements in the Conservative Party who want to reduce immigration levels and compel some Muslim Canadians to abandon their religious garb. Others continue to argue against protecting sexual minorities or the reproductive rights of women.

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Mr. Scheer must convince Canadians that his vision of conservatism embraces limited government, low taxes and a tough approach to crime, while also promoting Canada's multicultural reality. Mr. Trump and his politics of white resentment make that harder.

Later this month, NDP members will begin the first of what could be several rounds of voting for a new leader. Conventional wisdom (and I make no claims for its accuracy) holds that Greater Toronto MPP Jagmeet Singh and Northern Ontario MP Charlie Angus are the front-runners, though Quebec MP Guy Caron has picked up some prominent endorsements. Mr. Angus is (rightly or wrongly) seen as the candidate favoured by labour and other traditional NDP interests, while Mr. Singh is the candidate best positioned to help the NDP break through among immigrant and suburban voters.

Always, for New Democrats, the question is the same: Should we drift gently to the left of the Liberals, while assuring Canadians we are a responsible party that can be trusted with government? Or do we champion environmental and social activism and the working class, even if it makes us unelectable?

Neither approach has succeeded in bringing the NDP to power federally, though the former approach has worked, from time to time, in most provinces.

The wild card in all of this is the North American free-trade agreement renegotiation. Canadians generally support the Liberal government's efforts to protect free trade with the United States, while also striving to incorporate improved environmental and labour standards.

But what if the talks fail and Mr. Trump withdraws the United States from the treaty, leading to economic instability and uncertainty? Will Canadians blame Mr. Trump or Mr. Trudeau for the economic fallout?

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If they blame Mr. Trudeau, will they turn to the Conservatives as more reliable managers of the economy, or the NDP as more earnest champions of Canadian values?

If you think you know the answers to those questions, you should think again.