Prime Minister Stephen Harper called Peter MacKay a "historic figure" when the Conservative Justice Minister recently announced his departure from politics. But despite his starring role in uniting the right, the jury is still out on whether Mr. MacKay will be viewed as an agent of history, or a casualty of it.

What's left of the Red Tory faction that kept the Progressive Conservative Party alive after its 1993 electoral thumping still resents Mr. MacKay for betraying his promise to reject a merger with the Canadian Alliance after he won the PC leadership in 2003. But the truth is that, by then, Canadian voters had already condemned the PCs to the dustbin of history.

Mr. MacKay had only to heed their verdict. The unprecedented "sorting" of the Canadian electorate along right-left lines that began with the Reform Party's 1993 breakthrough triggered a process that no PC leader could have resisted.

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Until then, Canada's party system was one of the least polarized in the Western world. Two big-tent parties took turns governing based on how well each brokered vast regional and religious cleavages. Voters couldn't choose the Liberals or PCs based on their stands on state intervention or law and order because they were largely indistinguishable.

Canadian voters didn't suddenly discover their inner ideologue with the arrival of the Reform Party on the ballot. But, until then, the party system provided little outlet for ideological expression, despite occasionally successful attempts by the New Democratic Party to encourage class-based voting on the margins.

Using data from the Canadian Election Study, political scientists Stuart Soroka of the University of Michigan and Anthony Kevins of Aarhus University in Denmark correlated voter attitudes toward redistribution and party choice in every election since 1988. Their findings, outlined in a recent conference paper, suggest the "era of predominantly centrist politics may be over."

In 1988, PC voters outside Quebec "were if anything, a little to the left of the Liberals. There were clearly a good number of Red Tories. The development of the Reform, then Alliance, then Conservative Party produced a marked shift in the way in which parties captured and/or affected preferences for redistribution. As of 2011, Conservative voters [were] very clearly to the right of the other parties."

University of British Columbia political science professor Richard Johnston notes that PC victories up to and including 1988 depended on "radically incoherent" coalitions of voters. Canadians themselves came to realize this once voting for Reform became an option. With 2011's NDP wave in Quebec, the sorting trend under way in the rest of Canada since 1993 seemed finally to extend to that province.

The result is that "the big battalions of voters are now on the flanks, rather than in the middle," Prof. Johnston concludes. "Canadians' ideological locations are probably better sorted by party than they were 30 years ago, especially on the right."

Prof. Johnston argues that this sorting was "not ineluctable." But the way modern elections are fought suggests otherwise. Data-driven politics leads parties to aim for victory not by assembling "radically incoherent" coalitions of as many fundamentally incompatible voters as possible, but by mobilizing a much narrower base of like-minded voters. When barely 60 per cent of voters turn out, firing up your core supporters yields big returns on election day.

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In this respect, Canadian and American politics are converging. Fifty years ago, Republicans and Democrats aimed to win the White House with broad-based coalitions. Today, Blue Dog Democrats and Rockefeller Republicans are extinct as both parties target core liberal or conservative voters, squeezing out moderates.

In 1992, Democrat Bill Clinton fought for and won Kentucky, Louisiana, West Virginia and Tennessee. Today, Hillary Clinton has veered left to mobilize the Democratic base and won't even contest those states in 2016. The reason, as former Barack Obama campaign manager David Plouffe recently explained: "If you run a campaign trying to appeal to 60 or 70 per cent of the electorate, you're not going to run a very compelling campaign for the voters you need."

Big-tent politics is now all but dead here, too. The Tories and NDP are best positioned to profit from this development, provided Quebeckers stick to voting along right-left lines, not federalist-sovereigntist ones.

As for the Liberals, the centrist path to victory that was once such a sure thing is narrower than ever. If they lose in October, history may reserve for Justin Trudeau and his party the same fate it once held for Mr. MacKay and his PCs.