FOR all of the attention showered on hipster enclaves like Williamsburg, Brooklyn and Portland, Ore., America is only in the beginning stages of a historic urban reordering. After over a half-century of depopulation, cities have been filling up — and not just with young millennials, but with families and even older workers and retirees.

This reordering, should it continue, will have dramatic consequences for our politics and society — but only if the federal government undertakes its own historic reordering and shifts its priorities away from promoting the suburbs.

The influx of young people into cities is the biggest part of the story, and rightly so. The ranks of the so-called echo boom — the children of the baby boomers — constitute about 25 percent of the population. After nearly 100 years in which suburban growth outpaced urban, millennials are reversing the trend. Once only a fraction of young college graduates wanted to move to cities; now about two-thirds do.

In the past, many of those who moved to cities in their 20s moved to the suburbs in their 30s, where schools were good, crime was low and family-oriented amenities were plentiful. But those factors are changing in cities, too. Crime has remained low, while public schools and parks have been getting better in many places.