Allegheny County District Attorney Stephen A. Zappala Jr. is stitching together slivers of political coalitions into one he hopes will be robust enough to keep his own decades-old career alive.

He lost the Democratic primary to a progressive challenger in May, but an unusual write-in campaign organized by the county's top Republicans ensured the lifelong Democrat will have a place on the November ballot — albeit under the GOP's banner. Then, this week, the recently formed Forward Party — a loose centrist coalition that's not even registered as an official party in Pennsylvania — gave the longtime prosecutor its endorsement.

Forward Party CEO Lindsey Williams Drath said that by having won both Democratic and Republican primaries throughout his career, Mr. Zappala has shown his “cross-partisan appeal.” What Ms. Williams Drath referred to as “cross-partisan appeal” is not only a wise strategy but “it’s what you have to do,” Allegheny County Republican Committee Chairman Sam DeMarco said.

Particularly if your own party threw its support behind someone new — in this case Matt Dugan, formerly the county's chief public defender. Mr. Dugan is the latest candidate to capitalize on a nationwide movement, backed by some of the Democratic Party's biggest donors, to elect political progressives to powerful local prosecutor's offices. He beat Mr. Zappala, who's held the office since 1998, by more than 10 percentage points.