WASHINGTON—A growing number of GOP lawmakers have little incentive to come to work or toe the party line, putting critical legislation in danger as Republicans lurch toward the midterms. Some are privately calling it the zombie Congress. GOP leaders are increasingly concerned that members will lose motivation to show up, or may break with the party if they do, potentially stalling or killing some bills, according to lawmakers and aides. While the Senate has been the main focus of GOP cracks, the House is also facing an uncertain summer and fall, with attendance issues and a renewed sense of independence...