The most important number in the 2015 federal election will be 170, the minimum number of seats required for any one party to form a majority government.

On current polling trends, no party is anywhere close to reaching that mark. But what will it take for any of the three major parties to reach that magic number?'s current seat projection puts the Liberals and Conservatives in a virtual tie, with Stephen Harper's party in a position to win between 125 and 158 seats if an election were held today, only slightly more than the 114 to 152 seats Justin Trudeau's Liberals could win. The New Democrats, with between 43 and 77 seats, trail at some distance.

But these estimates imagine a scenario in which the final result differs only slightly from what the polls are currently saying. Widening these ranges to include a larger number of outcomes gives us a hint at how each of the parties could reach 170 seats.

The Conservative road to 170

The Conservatives would have the easiest time reaching that number, as their current maximum range tops out at 196. And the road map to 170 for the Conservatives is relatively simple: holding on to the regions they won in the 2011 federal election.

However, the Conservatives' support levels have shifted dramatically from one region to another since then. The party has, for instance, disproportionately lost support in British Columbia and Atlantic Canada while making significant gains in Quebec. So as we estimate what a Conservative majority could look like in 2015, we are better off to start at this vantage point than with the results of the last election.

It makes their route to a majority government run almost entirely through British Columbia and Ontario. The party's scope for gains in Alberta, the Prairie provinces and Atlantic Canada are minimal, whereas the Tories could get themselves over the 170-seat mark by winning the seats currently in play in B.C. and Ontario alone.

But theoretically, it appears the Conservatives could win a smaller share of seats in Ontario than they did in 2011 and still come out with a majority. The reason is Quebec. The party is experiencing a surge in support, enough to give it an extra 10 seats in the province — and in a majority scenario, perhaps as many as 15 more. That provides a cushion if the Conservatives lose seats in other parts of the country. 

On the map, a Conservative majority looks something like this: 20 seats in British Columbia, 50 in Alberta and the Prairies, 70 in Ontario, 20 in Quebec, and 10 in Atlantic Canada. While the tally in Quebec would be very high by the Conservatives' historical standards, the low target in Ontario gives the Tories room to manoeuvre.

The Liberal road to 170

With their current levels of support, the Liberals have far less margin for error. Their projected maximum range tops out at 171, just barely over the line. For them, every seat in every part of the country counts.

But nowhere are they more important than in Ontario and Quebec, which contain the vast majority of the seats liable to swing. Improving fortunes in the four western provinces might only boost the current projected tally of seats for the Liberals by as many as six.

In Ontario and Quebec, however, almost 30 seats currently projected to be won by the Tories or NDP could flip over to the Liberals if their support increases.

Nevertheless, gains in British Columbia, Alberta and the Prairies need to be made for the Liberals to get to 170. Without a significant surge in the two central provinces, a Liberal majority needs to include seats in B.C. as well as Calgary, Edmonton and Winnipeg.

The Liberal majority map would have about 20 seats in British Columbia, 15 in Alberta, the Prairies, and the North, 75 in Ontario, 35 in Quebec and 25 in Atlantic Canada. The map also requires the NDP to fall below the 50-seat mark.

The NDP road to 170

The New Democrats have been stagnating in the polls and are in no position to challenge for a majority government at this stage. Their maximum range goes only to 99 seats. However, that would maintain Tom Mulcair in his current role as the leader of the Opposition.

To stay there, the NDP's hopes lie almost entirely in Quebec. While the party, on current support levels, could still hold the majority of the 59 seats it won there in 2011, the NDP would need to hold virtually all of them to remain the second largest party in the House of Commons. The floundering of the Bloc Québécois makes that easier, but with the Liberals and Conservatives both polling substantially higher than the vote share they received in the last election, the New Democrats will need a new boost of their own.

British Columbia is another province where the NDP needs to hit double digits in the national seat count. Taking advantage of the more winnable riding boundaries in Saskatchewan is also important.

But if the NDP gets itself back into contention, what would a Mulcair majority look like?

British Columbia and Quebec would form the foundation, but a major breakthrough in Ontario would be necessary, as the gains the NDP could potentially make in Alberta, the Prairies and Atlantic Canada are limited. It would require something like 25 seats in British Columbia, 15 in Alberta, the Prairies, and the North, 55 in Ontario, 65 in Quebec, and 10 in Atlantic Canada.

The New Democrats are far from approaching that tally. But if voting intentions remain as stubbornly close as they have been for some months, the 170-seat mark may prove unreachable for any one party.'s vote and seat projection model aggregates all publicly released polls, weighing them by sample size, date, and the polling firm's accuracy record. Upper and lower ranges are based on how polls have performed in other recent elections. The seat projection model makes individual projections for all ridings in the country, based on regional shifts in support since the 2011 election and taking into account other factors such as incumbency. The projections are subject to the margins of error of the opinion polls included in the model, as well as the unpredictable nature of politics at the riding level. The polls included in the model vary in size, date, and method, and have not been individually verified by the CBC. You can read the full methodology here.