His decision opens a major opportunity for Democrats as they seek to recapture the House this fall. He represents a moderate suburban district that leans slightly right, but that went narrowly for Democrat Hillary Clinton for president in 2016. Until this week he was widely seen as strongly positioned to withstand what may be shaping up as a Democratic wave. His departure could also play into the court-mandated effort to redraw Pennsylvania's congressional maps, since Republicans will not have an incumbent to protect in the Delaware County-based district.