A skirmish broke out recently between and about the merits of single-payer health care, an idea that Mr. Sanders has long advocated. The candidates jousted about the financing of single-payer health care during the Democratic debate Saturday night. The differences between the two candidates on single-payer health care paint Mrs. Clinton as more moderate and Mr. Sanders as more liberal (no surprise there).

The question is: Do any Democratic primary votes turn on this issue? Single-payer health care and the Medicare-for-all...