Trudeau and Mulcair need to win big in their backyard of Montreal

This is thesixthin a series that will run until the end of the campaign,taking an in-depth look at where the polls stand in every region of the country and which seats are up for grabs. Check out the last instalment,wherethe spotlight was on Alberta.

The Greater Montreal region has never been a particularly decisive election battleground. For years, the area voted primarily for the Liberals on the Island of Montreal and for the Bloc Qubcois off of it. But after the dramatic changes the region saw in 2011, Montreal and the surrounding areas are no longer so electorally predictable.

The New Democrats won the Island of Montreal with 38 per cent of the vote in 2011, taking 14 seats. The Liberals finished second with 27 per cent and seven seats, while the Bloc Qubcois took 18 per cent and just one seat. The Conservatives had just 13 per cent support on the Island.

Off the Island, the New Democrats dominated with 47 per cent, winning every seat up for grabs. The Bloc trailed at a distance with 26 per cent.

But Quebec is in flux politically, and that is also the case for the Greater Montreal region. Seats that the Liberals would have struggled to win back are suddenly much more winnable as the NDP's support in the province slips. Long-shot hopes the Bloc might have had in a few ridings are no longer so unrealistic.

Projections for the region currently give the New Democrats a total of 20 seats out of 35. The Liberals are projected to win the other 15, all but three of them being on the Island of Montreal.

But there are many more ridings at stake in Greater Montreal. And though the battle is primarily between the Liberals and New Democrats, in some ridings the Liberals find themselves locking horns with the Conservatives, and in others the NDP has to hold off the Bloc Qubcois.

For the NDP in Quebec, defence is the order of the day. Hopes of making many new gains in the province have plummeted along with the New Democrats' poll numbers. But there are a few potential pickups in Greater Montreal for the NDP, thanks to the addition of three new seats in the northern and southern suburbs of the city and some close races against the Liberals in northern Montreal.

The following is a list of ridings that each of the partiescould pick up on election night. Favourable gainsare those in which there is a good chance of the party winning, potentialgains are those in which the results may be close and marginal gains are seats in which the party has an outside chance.

Favourable NDP gains:

• Two seats created north of Montreal.
• One seat created south of Montreal.
• Repentigny.

Marginal NDP gains:

• Ahuntsic-Cartierville.
• Bourassa.

The Liberals' main targets in Quebec are in western Montreal, a region that used to be one of the safest bets for the party anywhere in the country. Winning those seats that were lost to the NDP in 2011 in the parts of the Island with large non-francophone populations should not be too difficult, as polling throughout the campaign has shown a robust lead for the Liberals among this electorate.

Favourable Liberal gains:

• Ahuntsic-Cartierville.
• Brossard-Saint-Lambert.
• Dorval-Lachine-LaSalle.
• Honor-Mercier.
• Pierrefonds-Dollard.
• Ville-Marie-Le Sud-Ouest-L'le-des-Soeurs.

Potential Liberal gains:

• Alfred-Pellan.
• Vimy.

Marginal Liberal gains:

• LaSalle-mard-Verdun.
• LavalLes les.

A good night for the Liberals would seethem breaking into Laval. A very good night would see them winning all 17 ridings in which they are atplay in the region.

The floor for the Bloc Qubcois and Conservatives in the Greater Montreal region is zero, but they do have some potential pickups.

The Bloc's best chances liein eastern Montreal and on the South Shore, regions that are largely francophone. A good night might see their former (Mario Beaulieu) and current (Gilles Duceppe) leaders elected. But the Bloc's best hopes for gains are outside of the Greater Montreal region entirely.

Potential Bloc gains:

• Pierre Boucher-Les Patriotes-Verchres.

Marginal Bloc gains:

• Hochelaga.
• La Pointe-de-l'le.
• Laurier-Sainte-Marie.
• Montarville.
• Repentigny.

The Conservatives dreamed of a breakthrough on the West Island in 2011, and came close in a few ridings. Their chances are looking worse in 2015 with the Liberals polling much more strongly in Quebec. Nevertheless, there are three ridings in which the Conservatives have a (very) slim chance of taking on election night.

Marginal Conservative gains:

• Lac-Saint-Louis.
• Mount Royal.
• Pierrefonds-Dollard.

The 35 seats up for grabs in the Greater Montreal region may not play a decisive role in deciding who gets to form government, though both the Liberals and NDP need a good showing here in order to have a shot at 24 Sussex. If the NDP retains most of its seats, it may have a good night. If the Liberals cannot win in a region they have traditionally dominated, they are in for a rough showing. And if both parties find themselves dropping seats to the Conservatives and Bloc, they can almost be assured of spending another four years on the opposition benches.

CBC's Poll Trackeraggregates all publicly released polls, weighing them by sample size, dateand 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, dateand method, and have not been individually verified by the CBC.You can read the full methodology here.