This is the tenth and last in a series that has run throughout 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 British Columbia.

Long before this election campaign began, the Greater Toronto Area was identified as the key battleground in deciding who would be Canada's next prime minister. This is especially so in the region outside of the city of Toronto itself, and it has not disappointed. How voters in this volatile part of the country swing is perhaps the most important question on election night.

The outlying suburbs of Toronto famously swung towards the Conservatives in 2011, helping give the party a majority government. The Conservatives won 17 of the 18 seats on offer in the area, accounting for almost half of the net gains the party made compared to the previous vote in 2008. These seats were almost single-handedly enough to make the difference between a majority and minority government.

They may yet play the same role on Monday. Recent polls have shown a robust lead for the Liberals in the region, moving away from the earlier close race between them and the Tories. The New Democrats, who started the campaign with an outside hope of a few gains in the GTA outside of Toronto, have become an afterthought, dropping from the low 20s to the high teens.

Justin Trudeau's final push1:59

Riding polls have told the same story, and have highlighted some of the fiercest battlegrounds in the GTA. Some ridings in Mississauga, Markham and Oakville look like they will solidly go Liberal, but other ridings like Brampton Centre, Brampton North, and Mississauga–Lakeshore look like toss-ups between the Liberals and Conservatives. Decent NDP support in these two Brampton ridings in particular could play a key role in deciding the outcome.

Current projections award the Liberals the lion's share of the seats in the growing region (it has increased from 18 to 25 seats) with between 18 and 22 seat wins, with the Conservatives in range of winning between three and seven. The NDP looks likely to be shutout.

This could result in the disappearance of several key figures in the Conservative caucus from Ottawa: Chris Alexander in Ajax, Paul Calandra in Markham–Stouffville and Julian Fantino in Vaughan–Woodbridge. For their part, the Liberals are hoping to see the return of a few former MPs: Omar Alghabra and Navdeep Bains in Mississauga, and Mark Holland in Ajax.

But these voters being as volatile as they are, there are many more ridings which could potentially be at play.

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.

The Liberals are in play in almost every riding in the region. At a minimum, their strong polling numbers should deliver them most of the seats in Mississauga, Brampton, Markham and Oakville. But if the region really swings towards the Liberals, they could pick up seats north of Toronto, such as in Vaughan.

Favourable Liberal gains:

  • Ajax.
  • Brampton East.
  • Brampton North.
  • Brampton South.
  • Brampton West.
  • Markham–Stouffville.
  • Markham–Thornhill.
  • Mississauga Centre.
  • Mississauga East–Cooksville.
  • Mississauga–Erin Mills.
  • Mississauga–Lakeshore.
  • Mississauga–Malton.
  • Mississauga–Streetsville.
  • Oakville.
  • Oakville North–Burlington.
  • Pickering–Uxbridge.
  • Richmond Hill.

Potential Liberal gains:

  • Aurora–Oak Ridges–Richmond Hill.
  • Brampton Centre.
  • King–Vaughan.
  • Vaughan–Woodbridge.

Marginal Liberal gains:

  • Thornhill.

The Conservatives are playing defence in much of the country, but especially here. The biggest growth opportunity for the Conservatives is instead in the new ridings added to the region.

If the Tories get pushed out of the area, they could be reduced to just their seats in Thornhill and to the east of Toronto in Whitby and Oshawa. A better night would see them retaining their ridings north of Toronto, while a result that would likely indicate a Conservative plurality nationwide would see them holding the bulk of their seats in Brampton, Mississauga, Markham and Oakville. 

Potential Conservative gains:

  • Aurora–Oak Ridges–Richmond Hill.
  • Brampton Centre.
  • King–Vaughan.

Marginal Conservative gains:

  • Brampton South.
  • Markham–Unionville.

The GTA outside of the city of Toronto will not play an important role for the New Democrats. But they could pull off a few upsets. The NDP could benefit from a split in the vote to take Brampton East, while the party could steal Oshawa and its significant automotive industry due to the Trans-Pacific Partnership.

Marginal NDP gains:

  • Brampton East.
  • Oshawa.

But the real battle will be between the Liberals and the Conservatives. Its importance can hardly be overstated. Virtually every seat is up for grabs, and the Liberals are currently favoured to prevail in the vast majority of them. If they do, they will likely form the next government. But if the Conservatives can outperform expectations in just 10 seats (or even less) here, that could make the difference between a Liberal minority and a Conservative plurality, with all the constitutional implications that would likely entail. 

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.