America’s big cities are almost all dominated by the Democratic Party, but the politics of urban development are far from monolithic. In the past few years, a new faction has emerged across the country. Call them the new left urbanists.

These activists have big dreams. They want local governments to rebuild the urban environment—housing, transit, roads and tolls—to achieve social justice, racial justice and net-zero carbon emissions. They rally around slogans such as “ban all cars,” “raze the suburbs” and “single-family housing...