The head of Canada's opposition Conservative Party, who has a chance of beating Prime Minister Justin Trudeau in an election, on Saturday pushed back against charges he plans to weaken gun controls, an issue that could prove costly.

Conservative leader Erin O'Toole has been pressed repeatedly this week over his campaign promise to overturn a 2020 ban on weapons such as the AR-15, used by a gunman to kill 26 adults and children in the US 2012 Sandy Hook massacre.

O'Toole declined to...