This is the fourth analysis 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 southwestern Ontario.

Much rides on the outcome in Ontario in this federal election. Focus has been primarily on the tough battles in and around Toronto, but a number of ridings on the province's periphery will be closely fought as well.

The Conservatives did well in central and eastern Ontario in 2011, winning 21 of the 25 seats in the region. The Conservatives swept the rural areas, as well as taking four of the seven seats in and around Ottawa. The Liberals took just two seats in the capital, along with Kingston and the Islands in eastern Ontario. Only Ottawa Centre, the riding where Parliament Hill is located, went to the NDP.

Northern Ontario, however, was more fertile ground for the New Democrats. The NDP won six of the 10 ridings in the north, with the other four going to the Conservatives. The Liberals were shut out.

These three regions of Ontario are not likely to see many dramatic swings on Oct. 19. Current projections give the Conservatives eight seats in eastern Ontario, 12 in central Ontario, and three in northern Ontario, with the Liberals taking seven seats in the east and one in the north. No gains or losses for the NDP are currently projected.

But there is still plenty of time for things to change between now and election day. Though the Conservatives are unlikely to pick up any seats here that they did not win in 2011, they are in a strong position to hold their ground. They have nine safe seats in central Ontario and seven in eastern Ontario, and could be in play in as many as 24 seats in these two regions. In northern Ontario, however, the Conservatives could drop to between one and four seats.

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 the ridings, potential gains are those in which the results may be close, and marginal gains are seats in which the party has an outside chance of winning.

Favourable Conservative gains:

Three new seats added to central and eastern Ontario.

If the night goes badly for the Tories, they could be reduced only to the rural areas of central and eastern Ontario, being pushed out of Ottawa as well as Kenora and Sault Ste. Marie in the north. A good night, however, would see them holding on to most of the ridings they won here in 2011.

For the Liberals, a number of ridings in and around Ottawa should be high on the list of seats they could pick up on election night. Some rural ridings, particularly ones with large Franco-Ontarian communities like Nipissing-Timiskaming and Glengarry-Prescott-Russell, are also likely to swing over to the Liberals. A good night would see them pick up seats in the north and around Peterborough.

Favourable Liberal gains:

Nipissing-Timiskaming.

Orléans.

Ottawa West-Nepean.

Potential Liberal gains:

Glengarry-Prescott-Russell.

Kanata-Carleton.

Nepean.

Sault Ste. Marie.

Marginal Liberal gains:

Bay of Quinte.

Kenora.

Newmarket-Aurora.

Northumberland-Peterborough South.

Peterborough-Kawartha.

While northern Ontario has long been a stronghold for the New Democrats, the eastern and central parts of the province do not hold great prospects for the NDP. There is a good chance that the New Democrats will pick up a few more seats in the north (including the riding of Thunder Bay-Superior North, where they will face off against NDP-turned Green MP Bruce Hyer), but only if the party is on track for a majority government should they expect gains in central and eastern Ontario.

Favourable NDP gains:

Thunder Bay-Superior North.

Potential NDP gains:

Kenora.

Sault Ste. Marie.

Marginal NDP gains:

Ottawa-Vanier.

Peterborough-Kawartha.

Though the number of seats that might swing is not enormous in these parts of Ontario, many of the races are looking close. Sault Ste. Marie and Kenora in the north are setting up to be tight three-way battles, and Peterborough-Kawartha could also be on the target list for all three parties. In Ottawa, the Conservatives are facing stiff Liberal challenges in Nepean, Orléans, and Ottawa West-Nepean. Just because these regions may not decide the election (and with the national race so tight, even that cannot be said for certain), it does not mean they won't have their fair share of nail-biters.

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.