In the case at the center of the controversy, the Montreal-based SNC-Lavalin was charged with bribing officials in Libya to win contracts there, and defrauding the Libyan government when Muammar el-Qaddafi, the Libyan dictator, held power.

The prime minister said that in arguing for a civil penalty he was not trying to strong-arm Ms. Wilson-Raybould, but was acting out of concern for thousands of jobs in Canada. A criminal conviction would bar SNC-Lavalin from bidding on government contracts, a significant part of its business.

Although Mr. Trudeau faces no direct penalty as a result of the ethics commissioner’s finding, its release just weeks before campaigning begins for the October elections gives his adversaries plenty of ammunition. It also rekindles an issue that drew criticism from his own supporters and caused his Liberal Party to plummet in the polls, though it had largely recovered over the summer.

Royce Koop, a professor of political science at the University of Manitoba in Winnipeg, said Canadians generally respected nonpartisan officials like Mr. Dion, adding to the power of his findings. But Professor Koop said the timing of the report — in the middle of summer, when many Canadians’ minds are on vacation rather than politics — may work against it as a defining issue in the October vote.

Mr. Trudeau came to office in 2015 with great fanfare, as a new face with a new approach to politics — what he called “sunny ways.” He created a gender-balanced cabinet. He promised to push for protections against climate change while also protecting Canada’s energy industry. He said the country should reconcile with its Indigenous population, correcting historical wrongs.