Acronym, in use since the days of the ancient Roman Republic, for the Latin phrase: "senatus populusque romanum" ("The Senate and People of Rome").
The acronym has been used continually for approximately 23-24 centuries, and thus qualifies as the oldest acronym in current use.
Though it has been somewhat tainted by association with the fascist government of Benito Mussolini (which harked back to ancient Rome), it remains popular today. The city of Rome, proud of its heritage, still displays it on public works, most notably on manhole covers - despite the fact that the idea of a "Roman Senate" is completely out of date.