Democrat James Andrew Malone narrowly won a special election for a Pennsylvania state Senate seat in a stretch of Republican-leaning suburbs and farming communities, scoring an upset in a territory that a Democrat hasn’t represented in the chamber for 136 years.

James Andrew Malone, a small-town mayor, won a special election on Tuesday, narrowly defeating a county commissioner in a district the G.O.P. held for decades.

An upset victory in a state Senate race in Pennsylvania is the latest development to give Democrats hope of growing grassroots enthusiasm in the first few months of President Trump’s second term.