"I'm very pleased that Quebecers themselves have chosen to contest this bill in court, to stand up and defend the Charter of Rights and Freedoms," he said.
"I have been watching very closely the process, considering the federal potential actions, and at this time I feel it would be counterproductive for the federal government to engage in this process."
Quebec Premier Francois Legault urged the leaders to keep the law off the federal agenda, and to promise not to challenge it in court.
"Bill 21, which prohibits religious signs for people in authority, was adopted legitimately. A large majority of Quebecers support Bill 21. So I think Quebecers have the right to know. I would like the leaders of the federal parties to commit to not participate directly or indirectly, in any judicial challenge to this law."
While Trudeau has repeatedly said he is against the bill, Scheer -- kicking off his campaign in Quebec, where the measure is broadly popular -- dodged the question, repeating only that it was up to the courts and that a Conservative government would not introduce a similar federal bill.
Indeed, Scheer had more pressing matters to attend to Wednesday -- namely, siphoning votes away from a resurgent Bloc Québécois.
"It's not the Bloc that will replace Justin Trudeau, it's not the Bloc that will leave more money in your pockets," an energized Scheer told the crowd. "Quebecers can only rely on us, Bloc members of Parliament will always be powerless spectators."
Scheer kicked off his campaign energized by the rekindled SNC saga, a central element of the party's preferred narrative: that ethics scandals have robbed Trudeau of the moral authority to govern.
"You just cannot trust Justin Trudeau," Scheer said. "He will say anything to cover up his scandals, he will say anything to get re-elected."
For his part, Bloc Leader Yves-Francois Blanchet launched his Quebec-only campaign by urging voters tempted recently by the Liberals, Conservatives and New Democrats to return to their senses. The mission, he said, is not to win government, but to ensure the eventual government respects Quebec values of secularism, gender equality and love for the environment.
Scheer was to wrap up the first day of the campaign in the Toronto area, while Trudeau is holding his first rally in Vancouver, where he began and ended his run in 2015. Polls suggest the Liberals and Conservatives are in a dead heat, with the NDP and Greens fighting for a distant third.
The NDP launched their campaign Wednesday in London, Ont., one of the regions of the province, including Hamilton, where they think they'll be able to hold onto seats.
After a rousing speech to supporters, Singh quickly found himself fielding familiar questions about his party's meagre war chest, unfilled candidate spots and weak polling numbers.
Canadians, he said, are tired of governments that don't have their backs and will soon embrace the NDP's third way.
"I'm confident that people will see in us champions who want to put them at the centre and the heart of everything we do."
Elsewhere, some of the campaign's most intriguing supporting roles were looking forward to their own star turns.
"This is the most important election in Canadian history," Green party Leader Elizabeth May, her party buoyed by enduring concerns about climate change, told about 200 cheering supporters.
"We are going to stand firm. We will tell Canadians how serious the climate emergency is."
What climate emergency, wondered People's Party of Canada Leader Maxime Bernier.
"There's no climate emergency in this country," said the former Conservative MP and leadership runner-up, whose nascent party is running at about three per cent in the polls.
"We have the real policy that will help our country be freer and more prosperous," he said. "I'm not appealing to (Canadians) emotions, we are appealing to their intelligence."
At the dissolution of Parliament, the Liberals held 177 seats, the Conservatives 95, the NDP 39, the Bloc 10 and the Greens 2. There were eight independents -- including former Liberal cabinet ministers Jane Philpott and Jody Wilson-Raybould. The People's Party of Canada had one seat and former New Democrat Erin Weir sat as a member of the Co-operative Commonwealth Federation. Five seats were vacant.