VANCOUVER, British Columbia — The head of one of Canada’s major political parties tells a story about how at age 8, he decided to cover his head in line with Sikh custom and be called Jagmeet rather than “Jimmy” — a move that attracted the abuse of schoolyard bullies.

It also makes Jagmeet Singh, who leads Canada’s left-leaning New Democratic Party, stand out among candidates trying to unseat Prime Minister Justin Trudeau in Canada’s national elections in October.

Canada prides itself on multiculturalism, liberal values and openness to immigration. But Mr. Singh’s candidacy is historic: He is the first nonwhite contender to lead a major national political party and compete for the country’s top political job.

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“Just the idea of somebody like him who wears a turban leading a major national political party is a breakthrough in Canada,” said Brittany Andrew-Amofah, a leading Toronto-based policy analyst who is on the board of Urban Alliance on Race Relations.