MONTREAL — Four years ago Emmanuel Amoussa, a chemistry student, voted for Prime Minister Justin Trudeau of Canada, drawn by what he saw as a young progressive leader whose policies on climate change and immigration resonated with him and other members of his generation.

This time, though, Mr. Amoussa, who is now 22 and studies at the Université de Montréal, won’t be voting for Mr. Trudeau’s Liberal Party. Disappointed by the prime minister’s environmental policies and put off by recent revelations that he dressed in brownface 18 years ago, Mr. Amoussa said he planned to vote for the Green Party.

“In the last election Justin Trudeau seemed like a real change, but he has let me down,” Mr. Amoussa said this week during a lunch break from classes.

Mr. Trudeau swept to power in 2015, in part thanks to enthusiastic support from young people. But analysts said he could lose the election this time around if disenchanted young voters like Mr. Amoussa stay at home on Election Day, or split the vote by turning to another left-leaning party like the Greens or the New Democratic Party.