The Missouri Supreme Court on Thursday unanimously blocked newly enacted Republican-backed congressional districts from being used in November's midterm elections, ruling that voters must first approve the map in a statewide referendum.The ruling marks a rare setback for Republicans in a national redistricting battle. The GOP had aimed to use Missouri’s new congressional map to help them win an additional seat and hold onto its slim House majority.The court ruled that the referendum petition, filed in December after opponents submitted more than 300,000 signatures, prevented the new districts...