Donald Trump on Monday continued a potentially dangerous drumbeat — questioning the integrity of the American election system. These warnings are not new and not supported by evidence; they defy numerous studies that have found that voter fraud is minimal. They also invite a question: If the election is rigged, who is doing the rigging? Presidential elections are conducted on a state-by-state basis, not nationally. And in most of the states seen as presidential battlegrounds, the chief elections officers are Republicans — most directly accountable to their state’s voters.