PITTSBURGH, Pennsylvania — In politics, office-seekers form associations with movements and organizations in their bid to win elections. Those associations help them knit enough disparate groups together to form a coalition that will lift them to victory in a general election. But those associations can become problematic, especially if they were formed during a closed primary race in which candidates reach the furthest left or right to give them just enough votes to help cross the finish line. In those primaries, candidates are taking the chance that no one will care in a general election in which you have to...