“I deeply regret that I did that,” a shaken Trudeau told reporters on his election campaign plane late Wednesday. “I should have known better but I didn’t.”

The explosive photo of Trudeau, published by Time Magazine Wednesday, appeared in the yearbook of West Point Grey Academy, the private school where Trudeau taught at the time. The photograph shows Trudeau wearing a white turban, with his face and hands darkened. Trudeau, then 29, poses with four women, his brown arms draped over one of them.

The publication of the picture is a potentially crippling blow to Trudeau, coming only weeks before an Oct. 21 federal election. The Canadian leader, who has already seen his support wane over the past year amid an ethics scandal involving SNC-Lavalin Group Inc., has made diversity a central tenet of his governing principles.

“I’m pissed off at myself,” said Trudeau, who added he learned earlier Wednesday that the 18-year-old photo was going to be published. “I am asking Canadians to forgive me for what I did.”

Trudeau, 47, says he didn’t think at the time that dressing up as a dark-skinned “Aladdin” character for the gala was racist, but he realizes now that it was. “It was a dumb thing to do,” he said.

Trudeau has been a vocal supporter of women and minority rights since he was first elected in 2015. He was the first Canadian prime minister to appoint a gender balanced cabinet, and has boosted immigration levels to the highest since the First World War.

Canada’s election primer

“He has to answer to that,” said New Democratic Party Leader Jagmeet Singh, speaking to reporters in Toronto. “That’s really insulting” and makes a mockery of people and where they come from.

Aggregate opinion polling data compiled by the Canadian Broadcasting Corp. show Trudeau’s Liberals and the main opposition Conservatives in a dead heat for the election.