A handful of open Senate seats have drawn a crowd of Republicans vying to replace outgoing GOP senators. Those senators are mostly staying neutral in the primaries — at least for now. Alabama’s Richard C. Shelby is so far the only retiring senator to take sides in a primary, backing his former chief of staff, Katie Britt. While others weigh picking a favorite, the primary fields have continued to grow. In some races, vocal backers of Donald Trump are battling each other to succeed senators who have openly criticized the former president or have built reputations as legislative deal-makers. Two...