This is the seventh in 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, where the spotlight was on Greater Montreal.

The hopes of three parties lie in large part with the city of Toronto, where the Liberals, New Democrats and Conservatives all have a big number of seats to gain — or to lose.

Despite Toronto's reputation as a Liberal bastion, both the Conservatives and the New Democrats elected more MPs there in the 2011 election than the Liberals did. The two parties split 16 seats equally between them, with the Liberals winning the remaining six. And this despite the Liberals finishing first in the vote count, with 35 per cent to 31 per cent for both the Conservatives and NDP.

It was a big change in a city that had voted solidly Liberal since 1993. But Toronto could be about to swing back.

Polls suggest the Liberals hold a sizable and consistent lead in the city, with about 38 to 40 per cent support. The New Democrats follow with between 27 and 31 per cent, while the Conservatives have polled mostly between 26 and 28 per cent since the campaign began. Modest change, then, but enough to move a lot of seats.

Current projections give the Liberals the edge in 17 to 22 seats in Toronto, while the New Democrats are on track to win between three and six. The Conservatives could be shut out of the city, or prevail in as many as two.

ThreeHundredEight.com's projections for Toronto, as of Oct. 6, 2015. (Stephen McMurtry/ThreeHundredEight.com)

Riding-level polling suggests the race could be close in a number of ridings, with the Liberals and Conservatives battling it out for seats like Etobicoke Centre, Etobicoke-Lakeshore, York Centre, and Eglinton-Lawrence, and the NDP and Liberals locking horns in Scarborough Southwest, Spadina-Fort York, Toronto Centre and University-Rosedale.

For the Liberals to have a hope of forming government, however, they need to ensure the red tide about to rise on the shores of Lake Ontario is not held back.

Projections already award the Liberals the lion's share of the seats in Toronto, giving them the edge in 22 of the 24 seats in which they are in play (there are 25 seats in Toronto). So for the Liberals to form government, they need to win these seats. If they fall short in Toronto where they are currently expected to do well, they have no hope of surpassing the Conservatives in the national seat count.

Compared with 2011, gains for the Liberals are likely to come throughout Toronto: in the inner suburbs, particularly in the Don Valley and Scarborough. A good night would see them winning the seats they lost to the Conservatives in the outer rim of Toronto, and winning the NDP strongholds in the city centre.

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

Favourable Liberal gains:

Don Valley East.

Don Valley North.

Don Valley West.

Eglinton-Lawrence.

Etobicoke Centre.

Scarborough Centre.

Scarborough North.

Scarborough-Rouge Park.

Scarborough Southwest.

Willowdale.

York South-Weston.

Potential Liberal gains:

Beaches-East York.

Etobicoke-Lakeshore.

Parkdale-High Park.

University-Rosedale.

Marginal Liberal gains:

Davenport.

For the New Democrats, Toronto is key to their chances of winning on Oct. 19. Along with southwestern Ontario, it is one of the parts of the province in which the NDP was targeting for gains. The party is in play in as many as 11 ridings in Toronto, and would need almost all of them to help the NDP close the current seat gap with the Liberals and Conservatives. But the Liberal uptick makes that a tall order.

The low-hanging fruit for the NDP is in the old city of Toronto, where it hopes to win back Olivia Chow's former riding and win the new riding of University-Rosedale. A better night for the New Democrats would see them also retaining their gains in Scarborough, as well as making new ones there and in the city centre.

Favourable NDP gains:

Spadina-Fort York.

Potential NDP gains:

University–Rosedale.

Marginal NDP gains:

Scarborough-Rouge Park.

Toronto Centre.

Toronto would not be high on the list of regions for the Conservatives as they find themselves playing defence in most parts of the country. But the Conservatives have little choice but to aim for a majority if they are to retain power, and they do not win a majority without retaining the seats they won in Toronto in 2011, considering the losses they are on track to suffer in places like British Columbia and Atlantic Canada.

Put simply, Toronto is about holding what the Conservatives can in their stronger regions of Etobicoke and North York. But the party does have some slim prospects for gains as well, primarily in three-way contests in Scarborough, where the Tories could benefit from a divided opposition.

Marginal Conservative gains:

Don Valley North.

Scarborough-Guildwood.

Scarborough North.

Scarborough-Rouge Park.

Scarborough Southwest.

How the close races in Toronto go could play an important role in deciding who gets to form the next government. The Liberals look like they have Toronto, and absolutely need to keep it. The New Democrats look like they are on track for losses in the city, and desperately need to turn those around. And the key to a second Conservative majority is not only in winning the outer suburbs of the Greater Toronto Area, but also holding most of their seats in Toronto itself.

The NDP and Conservatives need a good night in Toronto. The Liberals need to make sure it won't be a bad one.

CBC's Poll Tracker 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.