FOR TOO LONG, it looked as if a small-time political squabble might unravel plans for hundreds of millions of dollars in construction along the best-known thoroughfare in the country -- Pennsylvania Avenue. It started last November, when Joseph B. Danzansky died and the charimanship of the powerful Pennsylvania Avenue Development Corporation, the independent agency overseeing plans and construction on Washington's main street, became vacant. Mayor Barry wanted attorney Max N. Berry; but Mr. Berry had been a member of a local dump-Carter group -- and guess who makes the official appointment?

So lo and behold, Messrs, Barry and Berry would find themselves alongside Rosalynn Carter at a full-blown press conference in January, extolling the virtues of the incumbent president. Then, if you can stand one more hitch, it turned out that the early-for-Carter local party chairman, Robert B. Washington Jr., was backing someone else for the corporation position. That led to an almost total stoppage of the agency's activities, since its rules make it difficult to make decisions without someone in the top office; an acting chairman was picked. Now that all this has been settled and a respected lawyer put in charge, official attention can again focus, as it should, on the corporation's reason for existing -- the enormous revitalization of the Avenue.

There will be big things happening. If all goes as it should, hotels, shops, offices, apartments, the old Post Office Building and even the Willard Hotel will be bubbling with life. That will be quite a contrast with the after-dark scene around the office buildings now; it's so quiet you can hear a paper clip drop. Not every detail of the plans will win universal praise, of course, but after much rethinking of the grandiose and heavy-handed design proposed in the 1960s, today's plans for a mixture of old and new architecture are activities as appealing as they are important to the vitality of downtown Washington.