OTTAWA — Prime Minister Justin Trudeau of Canada was buoyed into office by a wave of new and young voters. He will need those votes again if he is to win a second term in the upcoming national elections.

But that became more challenging on Wednesday after a scathing report by Canada’s ethics commissioner, who found that Mr. Trudeau broke the law by pressing his justice minister on how to handle a criminal case involving a multinational engineering company.

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The report is the first nonpartisan determination about Mr. Trudeau’s actins in the case, and was unequivocal in finding an ethics violation. It may end up influencing voters, political analysts said, if only by eroding enthusiasm for the prime minister among those who supported him in 2015, when he promised a fresh approach to politics.

“The Liberals face a challenge of getting these people who voted last time to come out and vote again,” said Andrew Steele, a former Liberal campaign strategist for Mr. Trudeau’s party, the Liberals.