Liberals leader Trudeau, who has apologized repeatedly for the photos, will appear in a live two-hour English encounter with the heads of the other five parties, most notably Andrew Scheer from the official opposition Conservatives.
English is spoken by two-thirds of Canada's 38 million population and the session has traditionally been regarded as crucial. The leaders will also face off in French, Canada's other official language, on Thursday.
"The official debates are going to be the big enchilada, perhaps more so than for any national debates in our history," said Frank Graves, president of polling firm Ekos. The debate starts at 7 pm Eastern Time (2300 GMT).
A Nanos Research poll for CTV and the Globe and Mail on Sunday put the Liberals on 36.7% and the Conservatives at 32.7%, in contrast to surveys that showed Trudeau falling behind after the blackface scandal broke on Sept. 18.
Trudeau started to pull ahead after Scheer stumbled through a French-language debate on private channel TVA on Wednesday. French is the main language in the populous province of Quebec, which accounts for 78 of the 338 seats in the House of Commons.
Scheer is also facing questions about why he did not reveal he has dual U.S. citizenship.
A Liberal official said Trudeau expected to be the main target during the debates.
A Conservative strategist said: "We're 100% focused on Trudeau." Scheer, though, may have a challenge landing a series of blows, given there will be six leaders on stage - the most in Canadian political history.
Ellie Alboim, who has helped four Liberal leaders prepare for debates, said the event would be a referendum on Trudeau.
"If the blackface incidents have indeed turned this into an election about character, voters who are still undecided will want to see how the prime minister fares," said Alboim, who works for government relations firm Earnscliffe.
Trudeau told French-language broadcaster Radio-Canada that he could not imagine a Conservative government."We're not perfect but we have done good work these last four years and we should be able to continue together to build a Canada that is more fair, open and productive," he said in comments shown on Sunday.