As the 2014 midterm campaigns come down the home stretch, Democrats are pounding on the issue of “equal pay for women.” In his speech at Northwestern University on Oct. 2, for example, Mr. Obama said that we must “make sure a woman is paid equal to a man.” Democrats are “for equal pay for equal work,” Hillary Clinton said at a recent rally in Iowa, “and our opponents are not.” North Carolina Sen. Kay Hagan’s campaign has blasted Thom Tillis, her GOP opponent, for opposing “federal equal pay legislation.”

As a campaign issue,...