Susan Lahey’s commanding lead in the York—Simcoe by-election market reflects her selection as the Ontario Progressive Conservative candidate in a riding long held by the party. The vacancy followed Caroline Mulroney’s resignation in June 2026, prompting Premier Doug Ford to appoint Lahey, an East Gwillimbury councillor and local business owner, in mid-June. With the September 3 vote approaching, traders appear to price in the structural advantage of PC incumbency and organization over challengers including Liberal Naomi Davison and smaller-party nominees. Late developments such as a major scandal, unusually low turnout, or coordinated opposition surge could narrow the gap, though no such shifts have materialized in the current cycle.