President Franklin Delano Roosevelt taught Democrats how to keep power by enacting bold programs that spoke to the pressing needs of working-class Americans. Unfortunately, the heirs to what is still referred to as “the party of FDR” have spent the better part of 75 years trying to unlearn that lesson.

This week, as Democrats toy with the politics of compromise and concession in a high-profile fight over spending priorities, they are once again breaking faith with what was best about FDR’s approach. At the same time, they are setting themselves...