Four federal leaders will take the stage in Montreal on Wednesday for the first debate to feature Liberal Leader Justin Trudeau.

Conservative Andrew Scheer, New Democrat Jagmeet Singh, and Yves-Francois Blanchet of the Bloc Quebecois will face off with Trudeau at 8 p.m. Eastern time, in a debate hosted by the private TVA television network and the Montreal newspaper Le Journal.

With the leaders spending most of the day preparing, it fell to Scheer's Quebec lieutenant Alain Rayes to kick off the 22nd day of this 40-day campaign with an event in Montreal, where he said the Conservatives would work with provinces and municipalities to clean up rivers.

Rayes did not offer any details but said the effort would include a focus on the St. Lawrence River, which served as a backdrop for his event and along which there are a number of seats considered up for grabs between Montreal and Quebec come Oct. 21.

Montreal dumped 8 billion litres of sewage into the St. Lawrence with federal government approval in November. The operation sparked public outrage even though the city said it had no choice because of needed repairs on its sewage system and insisted the dump would not affect fish or public health.

The Liberals were scheduled to hold their own announcement featuring cabinet minister Navdeep Bains in Toronto.

Trudeau was planning a visit later in the morning to a Montreal boxing gym for a photo op with trainer Ali Nestor in what has become a bit of a pre-debate tradition, while Singh is scheduled to go on a walkabout at a market, ostensibly to buy snacks for his preparation team.

The organizers didn't invite Green Leader Elizabeth May or the People's Party's Maxime Bernier, both of whom are campaigning in and around their home ridings for the day.

Wednesdays' debate is the first of three where Trudeau will take part, with two more next week organized by the new federal debate commission headed by former governor general David Johnston.

Trudeau didn't attend a debate hosted by Maclean's magazine and Citytv at the very outset of the campaign that's now half over, and the organizers of a debate on foreign policy cancelled their event when Trudeau wouldn't appear there, either.

All four of the leaders debating tonight will likely be hoping for moments that change the momentum of the campaign around its mid-point, with polls showing hardly any change in voters' preferences since it began on Sept. 11.

The Liberals and Conservatives remain locked in a fight for first place, hovering around 33 per cent support nationally, but with the Liberals apparently having a small edge because so much Conservative support is concentrated in the Prairie provinces. A strong Liberal showing in Quebec, where polls show them with a small but consequential lead, could sustain the Liberals in power; a strong Conservative showing could sink them.

The New Democrats remain well back, slightly ahead of the Greens in most polls. Without May on the stage, Singh will have a chance to sell himself to Quebecers in the hope of starting a new "orange wave" in the province.

This report by The Canadian Press was first published Oct. 2, 2019.

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