Sen. Bernie Sanders knows what he wants. In a Washington Post op-ed last week, the democratic socialist from Vermont laid out a legislative agenda for the 116th Congress’s first 100 days. Among the dizzying array of proposals Mr. Sanders pushed were his signature “Medicare for All” bill, tuition-free college, student-debt relief, tax increases requiring “wealthy people and large corporations to begin paying their fair share” and a $15 minimum wage indexed to “median wage growth,” not inflation.

Others in the Democratic Party’s...