One of the best lines of the U.S. presidential race so far comes from Wisconsin Gov. Scott Walker. “It’s sad to think right now,” he joked in the first round of Republican presidential debates, “but probably the Russian and Chinese governments know more about Hillary Clinton’s email server than do the members of the United States Congress.”

A cheeky zinger with a serious point: It would be alarming if, as U.S. intelligence veterans worry, Chinese, Russian and other hackers were able to steal U.S. secrets through Mrs. Clinton’s...