Nik Nanos is The Globe and Mail's pollster and executive chairman of Nanos Research.

Conventional wisdom holds that the federal Liberals are almost unassailable. After all, they lead in the polling by double digits, Prime Minister Justin Trudeau's personal scores are still high and the opposition parties are still transitioning to new leaders.

But today's numbers are not tomorrow's trends. Look at the U.K. election. Theresa May's Conservatives started off with a 21-point lead in the polls, and there was talk of a 100-seat majority. Labour Leader Jeremy Corbyn was derided and dismissed. The Conservatives' lead has narrowed significantly, and there is no longer much talk of a big win. There are lessons there for Canada on a number of fronts for both the Liberals and the New Democrats.

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First, the progressive left is getting squeezed in democracies around the world. In the U.K., Mr. Corbyn's strategy was to veer decidedly to the left. His "#WeDemand" advertising campaign is beyond traditional, moderate progressive politics, embracing a more left-wing populism in which people left behind in a changing economy lash back at the establishment.

Although many political observers have focused on the rise of the right, there has been a corresponding rise of the left in elections. In the United States, Bernie Sanders ran an insurgent campaign within the Democratic Party against Hillary Clinton. In the French presidential elections, far-left candidate Jean-Luc Mélenchon did well in the first round – a mere four points behind eventual winner Emmanuel Macron. And in the Dutch election, both the far left and the far right picked up support.

The first glimmer of this shift to the far left was in Greece. Pasok, a progressive social democratic party and one of the two traditional ruling parties, went from winning elections with more than 40-per-cent support to being replaced by the further-left Syriza party. The key takeaway is the phenomenon now called "Pasokification," in which mainstream progressive parties are getting squeezed as voters opt for either left-wing or right-wing anti-establishment politicians.

This polarization likely points to one possible path for Canada's New Democrats. With many of their progressive policies co-opted by the Liberals, the NDP presenting themselves as another moderate social democratic movement could lead them to political oblivion in the short term. One option for the NDP is to veer left of the Liberals, reclaim the New Democrat populist heritage and attack U.S. President Donald Trump.

Adding a hard anti-Trump edge to their policies would be a clear political opportunity. The Liberals are hamstrung by the fact they are in government and need to balance Canada's economic interests with maintaining a stable and cordial relationship with the Trump administration. There are elements in the Conservative Party who favour parts of Mr. Trump's tone and focus. This leaves the field open for an anti-Trump left-wing movement. As a polarizing figure in world politics, the U.S. President activates both the right and left in one stroke.

In an environment with a resurgent anti-Trump NDP and a renewed Conservative Party, grounded in fiscal conservatism and social values, the Liberals could very well be squeezed in the middle by trying to appease Canadians but not satisfying anyone. Today's comfortable Liberal lead in the polls could turn quickly ahead of and into the next federal election. Governments with big leads in the polls can get complacent and arrogant. Elections are times for governments to account for their actions and to demonstrate how Canada is a better place under their leadership.

In 2015, Canadians wanted change and voted for it. The Liberals clearly delivered on a different tone and style of politics. In the next election, they will not only have to account for what they have delivered as a government, they may be caught fighting a two-front war. Canada is not immune to the populist polarization taking place around the world. All parties should take note.