On Wednesday, the riding of Sarnia–Lambton received visits from two party leaders — Andrea Horwath of the New Democrats and Doug Ford of the Progressive Conservatives.

It is a sure sign that southwestern Ontario will be an electoral battleground in next week's provincial election.

Sarnia–Lambton voted for the PCs in 2014, but it's just one of several seats in the region up for grabs. Among them are all the ridings that went Liberal four years ago.

Southwestern Ontario used to be Liberal bedrock, but it has been moving away from the party in recent elections. The Liberals captured 41 per cent of the vote in the region in 2007, but dropped into second place behind the PCs with 34 per cent in 2011. By the 2014 election, the Liberals were down to third in the southwest and just 28 per cent.

That left them with only five seats in the region. They could lose them all next week.

The polls suggest that Liberal support in southwestern Ontario has collapsed. The party has just 11 per cent support there in the May 30 update of CBC's Poll Tracker, an aggregation of all publicly available polling data.

(The Poll Tracker is updated daily and the numbers can change quickly — check out the latest projections here.)

The New Democrats — who entered the campaign holding six southwestern ridings — hold a narrow lead there with 42 per cent, up 11 points from where they finished four years ago. The PCs (the incumbents in the remaining 10 ridings) are not far behind with 39 per cent, up six points since 2014.

Ontario Progressive Conservative Leader Doug Ford was also in Sarnia–Lambton on Wednesday, meeting with PC candidate Bob Bailey and supporters. (Geoff Robins/Canadian Press)

The Greens have six per cent, while two per cent of voters say they will vote for another party. Of those small parties, only the Libertarians are running candidates in a majority of ridings in southwestern Ontario.

Southwest helping to drive NDP's surge

The NDP's prospects in the southwest have improved along with the party's provincewide numbers. The New Democrats began the campaign trailing in southwestern Ontario by about 12 points; at the time they were in a position to win between four and nine seats there.

But the party's 10-point gain in the polls in just a few weeks has transformed the electoral map. New Democrats could now win between eight and 13 seats in the region — matching the PCs' potential.

Subscribe to the Pollcast podcast

Because the Liberals won their seats in the urban areas of the southwest — Cambridge, Guelph, Kitchener and London — the New Democrats are better positioned than the PCs to pick them up.

But New Democrats aren't merely poised to pick up a few Liberal seats in the region. They could take a run at a few PC seats as well.

Blue vs. orange

There are two types of electoral contests shaping up in southwestern Ontario.

On the one hand, there are the battles between the PCs and the NDP in seats that used to be Liberal — ridings like Cambridge, Kitchener Centre, Kitchener South–Hespeler and London North Centre.

On the other hand, there are seats that the New Democrats will be looking to take from the PCs. These are located primarily between Windsor and London and include Chatham-Kent–Leamington, Sarnia–Lambton, Elgin–Middlesex–London and Lambton–Kent–Middlesex, as well ridings further afield such as Huron–Bruce and Kitchener–Conestoga.

Earlier in the campaign, when the PCs were stronger in the polls, the NDP might have been vulnerable in its own seats of Waterloo and London West. That may no longer be the case.

And then there's Guelph. Under normal conditions, it likely would swing over to the NDP along with the other Liberal ridings. But the candidacy of Green Party Leader Mike Schreiner there makes the riding more unpredictable. If Schreiner can replicate the breakthrough Green leaders have pulled off in British Columbia, New Brunswick and Prince Edward Island, he may be one of the surprises on election night.

The Ontario election in miniature

Nowhere else in Ontario are Kathleen Wynne's Liberals polling worse than in southwestern Ontario. That makes the region a microcosm of the entire election — with the PCs dominant in the rural parts, the New Democrats well-positioned to pick up urban Liberal seats and the overall winner to be decided between the two.

In order to form a government, the PCs will need to sweep the rural parts of the province and add enough urban and suburban seats to their total to put them over the top. For the New Democrats, forming government means sweeping urban Ontario and winning enough suburban and rural seats to put them in the premier's office. Southwestern Ontario is just one piece of that puzzle.

There aren't enough seats in southwestern Ontario to make the region decisive on its own. But winning in the right places there means winning in the right places in the rest of the province.