MONTREAL — The Liberal conversion to electoral reform is both a reflection of the party’s diminished circumstances in the House of Commons and — potentially — a watershed moment in Canadian electoral politics.

For the first time, two of Canada’s three main national parties — the NDP and the Liberals — are about to run on a commitment to do away with the first-past-the-post voting system to conduct federal elections.

They may not agree on the fine print. The New Democrats have long championed a mixed proportional system and the Liberals tend to be partial to a preferential ballot. But they are on the same general page.

On Tuesday, Justin Trudeau pledged to introduce a new voting system in time for the 2019 election.

The New Democrats have advocated a more proportional voting regimen for decades.

With the Green party on side, the blocks of a large parliamentary consensus on electoral reform are falling into place.

It took a change of heart on the Liberals’ part to bring that about.

Such a change would have been unthinkable back when the party, under Jean Chrétien, was riding high and reaping the benefits of the first-past-the-post system against a divided right.

Even after they lost power to Stephen Harper’s Conservatives a decade ago, the Liberals continued to believe that the normal swing of the pendulum would inevitably see them back in power.

Three lost elections later they are at least as likely to remain in third place as to form a government this fall.

Implicit in the party’s shift on electoral reform is a Liberal admission that the NDP’s competitive position is not a mere blip but rather a feature that it will have to adapt to.

In a three-way battle the current winner-take-all-system favours the Conservatives.

That conclusion is only belatedly dawning on Trudeau’s party.

It has been more than three decades since the Liberals last won a governing majority against a united Conservative party.

Even with the wind in their backs, the New Democrats would also be hard-pressed to eke out a majority in October.

Win or lose next fall, electoral reform may offer both parties the best alternative to a merger at some point down the line, and — more importantly in the big picture for each of them — their best shot at preventing the Conservatives from becoming Canada’s de facto governing party.

Trudeau’s democratic reform package may not do much to turn the tide in his favour. Paul Martin and Michael Ignatieff tried and failed to impress voters with measures to fix the so-called democratic deficit in Parliament.

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The excitement that electoral reform inspires among voters tends to be inversely proportional to the hype that surrounds the issue within the chattering class.

In the provinces where a plan for a different system was put to voters, the status quo prevailed.

But the expanding common ground between the opposition parties on electoral reform should be cause for concern for the Conservatives.

They would — at least initially — be the losers of a switch to a different electoral system. The division in the opposition vote has long been a key ingredient of their election victories.

It allowed Brian Mulroney to win a second majority mandate and the 1988 free trade battle.

Under Stephen Harper, a divided opposition vote has become even more essential to Conservative fortunes.

He is about to lead the party in his fourth election campaign as the incumbent with a persistent shortfall in support and precious little room to grow.

In a preferential voting system — where the second and third choices of voters are applied in each riding until a candidate reaches 50 per cent plus one — Harper’s re-election bid would be dead on arrival.

Alternatively, under a mixed proportional system, securing a majority government on a bit more than a third of the votes cast would become a thing of the past. Coalition building would not come easily to the take-no-prisoner party that Harper has rebuilt in his image.

Should the NDP or the Liberals or a combination of both prevail next fall, they would have a common interest in following up on electoral reform.

Chantal Hébert is a national affairs writer. Her column appears Tuesday, Thursday and Saturday.

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