If you haven’t heard, much of the nation’s capital now reeks of sewage — and there’s a damning reason you likely didn’t know.

The January collapse of a major sewer pipe outside DC — spilling 250 million gallons of poop into the Potomac River — is the latest demonstration of how Democrats can screw up the most basic function of government, no matter how much money and resources they have at their disposal.

The failure of DC Water’s Potomac Interceptor line is now the subject of finger-pointing between all the parties to the interstate compact: Maryland, Virginia, DC, the various localities, the National Park Service, etc.

But the real question is why the richest region in the nation — including Loudon and Fairfax counties in Virginia and Montgomery and Prince Georges counties in Maryland, as well as the nation’s capital — has the wastewater infrastructure of a Third World slum.

The failure of the 65-year-old Potomac Interceptor was not only predictable — it was predicted: Last September, DC Water announced it was beginning an “effort to rehabilitate” the line “before its condition deteriorates further.”

“Deteriorates further”? How bad was it, and how did it get that way?

The hallmark of a technologically advanced, wealthy and functional society is maintenance, or fixing things before they reach the point of imminent failure. Waiting until things break and then flinging blame around is how failing societies with no sense of civic responsibility behave.

Maybe the problem comes from the top: DC Water chief Unique Morris-Hughes has a long history of work in public administration, largely in human resources and “workforce development” — not in infrastructure management.

In her stint running the DC Department of Employment Services, Morris-Hughes was dinged for invoicing the city for tens of thousands of dollars in “questionable” entertainment and vacationing in tony spots with a major contractor.

It’s hard to escape the impression that DC Water — like so much of America — is drifting along on the fumes of yesteryear’s hard work, and eating the seed corn we’re supposed to be saving for the future.

When political favoritism and special-interest feeding outweigh competence, which is a given in all too many bright-blue regions of the country, it should be no surprise when things fall apart: Just look at the shoddy state of so many roads in New York and New Jersey.

The progressive Democrats who run most of urban America love big government, but ensuring that government takes care of the essentials is another matter entirely.