American families are enjoying a level of economic relief they haven’t experienced in six years—and it is largely attributable to the recent slide in gasoline prices. The average price for a gallon of gas, according to AAA, has tumbled 34% in the past year to about $2 a gallon from $3.27. Families are saving $100 a month at the pump and are paying less for groceries and everyday goods thanks to lower transportation costs. Considering that median family incomes are 3% lower than they were six years ago, this is a welcome reprieve for the middle class.

Politicians,...