The longest-running show in town has the least surprising plot. The actors come and go but the roles remain more or less the same. The motives often are unclear, or at least unspoken. And the end sometimes seems in sight, but never comes. This is, of course, Washington's West Side Story, the redundant drama of the Capitol West Front, a hit in spite of itself.

In this curious story the principal issue--the fate of the oldest existing fac,ade in the nation's most symbolic structure--usually is nothing more than a passive backdrop for the clash of petty interests, strong wills and increasingly esoteric arguments. Characteristically, last week, when something seemed about to happen, it didn't.

The American Institute of Architects, which has opposed expansion of the West Front for five decades, had geared itself for yet another struggle up on the Hill, this one to culminate with atypical precision next Tuesday, the date of the next scheduled meeting of a little-known six-member body called the Commission on the West Front of the U.S. Capitol, and, coincidentally, the 125th anniversary of the institute itself.

Failing to comply with this scenario, the commission postponed its meeting. Meanwhile, the either-or issue remains the same: whether to restore the original west fac,ade or to add to it, creating a new, larger look for the Capitol where it faces the city's most inspiring view of Pennsylvania Avenue and the Mall.

While the stalemate continues the old West Front stands in a disgraceful state of disrepair, its painted sandstone face still flecked and cracking, its portico columns still encumbered by wooden braces theatrically placed there years ago by J. George Stewart, a notoriously expansion-minded architect of the Capitol. George White, the current architect of the Capitol, with the support of the speaker of the House, also has been pushing hard for expansion.

White's plan is smaller than Stewart's. It would allow the platform and cascaded stairways designed by Frederick Law Olmsted to remain. But its logic is just as questionable. He says, for instance, that his intention is to preserve the west wall by stablizing it with a new structure, so that it would simply become an interior rather than exterior wall. As Robert M. Lawrence, current AIA president, aptly responded, "Entombment is not historic preservation."

White and others also justify expansion with the argument that the building has evolved over time, first with Thomas U. Walter's flanking House and Senate wings and his cast-iron dome in the 1850s and 1860s, again with Olmsted's marvelous park system later in the century and then with the expansion of the east fac,ade in the late 1950s. This argument has sent opposing camps back through the documents in an attempt to clarify the intentions, concerning expansion, of both Walter and Olmsted.

Predictably, they reached opposing conclusions, but so what? Walter and Olmsted may have considered the possibility of expansion. Happily, they were not forced by the Congress actually to engage in it. Expansion today is no less a defacement of a significant piece of history than it would have been a century or half a century ago. And one look at a floor plan of White's proposed expansion demonstrates that it represents no tiny change to the West Front. It would fundamentally alter the deep, sculpted massing of the building and from close up, on Olmsted's lawns and terraces, it would obscure Walter's magnificent dome.

But if the plan is esthetically and historically the wrong thing to do, it clearly responds to the space needs of the Congress, both real and imagined, and to the mood that constantly provokes the House leadership to look enviously across at the tucked-away offices provided for their Senate colleagues and to say, in effect, "We want some of that and, history be damned, we're out to get it."

These are truly little things to anyone removed from the psychic confines of the Capitol. It almost defies belief that the Congress cannot find the ingenuity to solve these problems without resorting to historic desecration, especially after the publication last fall of the long-awaited master plan for the entire Capitol precinct. Ironically, the creators of this plan, although specifically forbidden to consider the West Front issue (long-term opponents of expansion were suspicious that the master plan could be used as a cover to justify that result), ingeniously managed to suggest an answer to it.

Along with their admirable proposal to put parked cars in a garage underneath the east plaza--Olmsted's beautiful front yard for the Capitol--they offered quite a bit of room "as a future means of providing additional service and possible office space in close proximity to the Capitol." Legislators afraid that underground space would be beneath their dignity should be assured that it is not an irresolvable architectural problem.

Which brings us back to the Commission on the West Front of the U.S. Capitol, which used to be called the Commission on the Extension of the West Front of the U.S. Capitol. Despite the healthy change in name, this body, consisting of the vice president and the majority and minority leaders of each House, has done precious little besides reflecting an existing stalemate. The gentlemen of this commission should be urged to meet, to refuse to play conventional trade-off politics with the precious sliver of national history entrusted to their care and bring the story to an overdue happy ending.