With Republican-appointed judges reshaping the federal courts, Democrats have described legal conservatives as partisans who want the same things and think the same way. Close Supreme Court-watchers know this is false. Last term there were more 5-4 decisions when a conservative joined the liberals than when the conservatives were united. But if there were still any doubt, Monday’s ruling requiring unanimous jury verdicts offers an illuminating look at the diversity of conservative thought.

In 48 states, a conviction in court...