THERE IS NO set standard for precisely when people become "institutions," but other people can sort of tell. And for those who call Washington their home town -- and who hear so much about how transient everybody allegedly is here -- there is deep and abiding local lore to be mined and cherished at a certain left-hand side of the AM radio dial. No question: after 25 years together in their prime of radio time -- mornings, that is, on WMAL Radio 63 -- Frank Harden and Jackson Weaver are a certified local institution, not to mention the most popular one on the air.

Yesterday, these two sunrise loonies turned into quite a special event too: Some 4,000 to 5,000 well- wishers from every corner of Greater Washington hopped up in the pre-dawn hours to pack the Grand Foyer of the Kennedy Center, where Harden & Weaver did their anniversary broadcast. President Reagan and Vice President Bush joined in by tape, and celebrities from Hollywood to Wisconsin Avenue dropped by to reminisce, cajole and congratulate. And when a television station covers a radio show live -- for a full hour in the capital city, as WJLA-7 did -- you're not talking bush-league star-worship. So what is it that has kept such a huge and fiercely loyal following glued to this same show on the same station for such a record-breaking time? The format, such as it is, is old-time radio, for sure; the best archaelogists in the world couldn't dig up older jokes. It's pure corn, frittered in banter. Together with all the characters they have created over the years, Mr. Harden and Mr. Weaver still trot out the polkas, marches, hymns, poesies and homespun huckstering that was the standard fare when they first teamed up.

But they are more than entertainers, and therein lies a secret to their "institutionalization": Besides abiding each other with apparently genuine relish for a quarter of a century, Mr. Harden and Mr. Weaver have contributed phenomenal portions of their off hours to civic and charitable activities, bridging the generations and building the generosity of Washington along their routes.

For all their morning glories, and for making this town happy about itself and its traditions, Harden & Weaver have earned the local affection they enjoy.