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If the current poll results worry Conservative Leader Stephen Harper, another number represents his biggest obstacle: 170, the number of seats it takes to win a majority in the Commons. The politics of the next minority Parliament will make it hard for him to hold onto power, even if he has the biggest block of seats.

The leaders of the other parties are lining up to say they'd vote to defeat Mr. Harper right from the get-go. And it's not just talk. For most, if not all, backing Mr. Harper would be political suicide.

There's another potential outcome that's unlikely, though it's been hotly debated among those opposed to Mr. Harper: a coalition.

Yes, it's perfectly constitutional, and yes, many people tell pollsters they like the idea. But it's not likely to be in the political interests of those who'd have to do it. A garden-variety minority government is much more likely. If the election results were to mirror the latest Nanos Research poll, it would be a Liberal minority.

There are constitutional conventions about how minority governments are formed. But politics will dictate how it plays out.

Start with the rules: Canadians vote for MPs, and prime ministers must be able to win a confidence vote from a majority of them. If no party wins a majority of seats in the Commons, there's a kind of process of elimination to see who can command confidence.

Immediately after the election, the prime minister, in this case Mr. Harper, remains prime minister, whether he finished first or last. No matter how many seats he has, Mr. Harper could meet the Commons and try to win a confidence vote. A PM who is doomed to lose typically resigns before that, but either way, if he can't muster a majority in the Commons, he's out.

That doesn't mean a new election. The Governor-General would then look to see whether another MP can command confidence in the Commons, usually the leader of the opposition party that won the biggest block of seats. That leader could either try to form a coalition, inviting members from another party into the cabinet, or just form a minority government, seeking support on votes from one party or another.

But the politics narrow down the options.

First, Mr. Harper has a weak chance of forming a minority government. He's done it twice before, but this election campaign is a polarized Harper-versus-anti-Harper affair. Politicians on the other side know what their voters want.

Liberal Leader Justin Trudeau has said there's no way he'd prop up a Conservative government. NDP Leader Tom Mulcair has, too. So has Bloc Québécois Leader Gilles Duceppe.

Those campaign statements could be bent after the votes are in, but the leaders would face a backlash. They know there's a powerful, pent-up desire among their own supporters to oust Mr. Harper.

For Mr. Trudeau and Mr. Mulcair, polls show half of each party's supporters pick the other as second choice, and many will switch to the one who can beat the Tories. The leader who keeps Mr. Harper in power risks being abandoned by them, and decimated in the next election, which could come quickly.

Perhaps Mr. Harper could eke out a deal with a weak opposition party if he's just a few votes shy of majority. But otherwise, it's likely majority or bust.

That means a minority Parliament would probably bring a different government. But it's not likely to be a coalition. Coalitions can be stable governments, but the politics are tricky.

Imagine, for a moment, if the results did mirror the latest polls. Mr. Trudeau's Liberals, in first, don't really want to share power with the NDP, in third. And they would know the NDP can't support Mr. Harper, so it's virtually compelled to prop up a Liberal minority. Maybe the two would agree to exchange policy concessions for guaranteed support, but maybe not.

The third party doesn't have much incentive to join a coalition, either. Junior coalition partners share blame but not credit, and often lose in following elections. It's a much better gamble for a third party to prop up a new, untested government, and bring it down when the time is right.

It means Mr. Harper, running second in polls, has to do more than catch up. If he's short of a majority, the politics of a minority Parliament are likely to defeat him.

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