ONE OF THE longest stories on Pennsylvania Avenue is the effort to preserve, repair and renew the 81-year-old Post Office Building at 12th Street NW, a marvelously appealing architectural oddity that combines arches, dormers, turrets, columns and other trimmings. When we last looked in on it, the rescue mission seemed to be going quite well. Not only had the White House, Congress and every other interested party given the project appropriate and necessary blessings, but workers were all over the building, sprucing and shoring it up as part of a grand plan to rejuvenate the entire Pennsylvania Avenue effort.

Well, don't look now. This could be the start of something big but not quite finished. Because of some unanticipated structural work, coupled with other alterations that had to be made to accommodate a gift of bells from Britain for the building's tower, Washington could wind up with another huge white elephant unless the White House acts in the next few days to keep things moving. It's not a matter of megabucks, either, but of approval of already available funds to wrap up the project on time.

The building is supposed to be in shape in about a year. But without a go-ahead for the final stages of work, this centerpiece of the new Pennsylvania Avenue will be a boarded-up salute to plywood. And if past depressing experience is any guide, the effects of inflation will have put any more work out of sight instead of out of pocket. Having come so far after so long, it would be terrible if the old Post Office became just another monument to government penny-wisdom.