Republicans were heading toward losses in two U.S. House seats they already controlled and appeared on track to flip no more than one of their gerrymander targets.

Last summer, the Republican-controlled Texas Legislature gathered for an impromptu special legislative session to—on direct orders from President Donald Trump—redraw the state’s congressional maps to give Republicans an edge in five more seats in the U.S. House. These redraws were, by and large, centered on the…