One of the cardinal rules of Ontario politics has long been that it is virtually impossible to win an election without taking most of the seats in the suburban areas that surround Toronto, the region known as the 905.

This truism is even more applicable to the 2018 vote, where a new electoral map boosts the number of ridings in the 905, made up of Halton, Peel, York and Durham regions. It now accounts for 29 seats, nearly one-quarter of those up for grabs provincewide.

The 905 will be "extremely important" to the election outcome, said Quito Maggi, president of the polling firm Mainstreet Research.

The 905's influence on the overall seat count is one of the key reasons why pollsters and the CBC's Poll Tracker suggest that if the two parties finish neck-and-neck in the popular vote, the PCs under Doug Ford would be more likely than the NDP under Andrea Horwath to win the election.

NDP Leader Andrea Horwath has made multiple campaign stops in Brampton. (Cole Burston/Canadian Press)

The NDP won just two seats in the 905 in the 2014 election: Oshawa and Bramalea-Gore-Malton. The New Democratic candidates finished third in every other race in the 905, with the sole exception of Brampton-Springdale.

It means the NDP would need a huge swing of votes in its direction to overtake the Progressive Conservatives and the Liberals to dominate the region.

Conversations with campaign officials from all the parties suggest, based on current polling, the Liberals face a tough test to hold on to any seat in the 905, unless their numbers surge in the coming week.

The PCs admit the NDP is strong in Brampton and in a few seats in Durham region, while the NDP acknowledges that the PCs are the party to beat across York region and Halton region.

The races appear to be close in four of Mississauga's six seats, but favouring the PCs.

Liberal Leader Kathleen Wynne campaigned Tuesday in Oakville, a riding that has been won by Liberal Kevin Flynn, right, in four straight elections. (Nathan Denette/Canadian Press)

That could add up to a scenario in which the PCs take more than 20 of the 29 seats in the region. If the polls swing further against the PCs, however, the New Democrats could take seats in parts of the 905 where they've never been successful before.

"Primarily, it depends on the Liberal vote collapsing," said Maggi. "It's going to come down to a handful of extremely close races, and more than half of them are in the 905."

The winning party in the last six straight elections also took the most seats in the 905. In the elections that produced a change in government, the Liberals under Dalton McGuinty took 12 of the 18 seats up for grabs in 2003; the PCs under Mike Harris swept all 18 in 1995.

However, 1990 provides the most recent exception to the rule that you have to win the 905 to take power in Ontario. Bob Rae's surprise NDP victory came primarily from the party's dominance in Hamilton-Niagara, the northern and southwestern parts of the province, and its wins in Toronto. The NDP took just six seats in the 905 that election, while the Liberals won eight and the PCs four.