To architectural preservationist Judy Sobel, the downtown blocks of F Street NW are living evidence of how Washington's commercial core has evolved in the last 150 years.

Washington's retail center grew up along the F Street corridor, filling in the blocks between the Patent Office at 7th Street and the Treasury building at 14th Street after those two buildings were erected in the 1830s.

It was along F Street that dry goods merchants matured into the District of Columbia's first department stores. The prosperous proprietors of F Street put up new-fangled outdoor lights in the 1880s. Rustic storefronts gave way to the trendiest buildings of the late 19th Century.

Most of those century-old stores still are standing. Hidden behind 20th Century plastic facades and neon nameplates are blocks of Italianate, Romanesque revival and beaux arts buildings that once again are becoming the most important commercial district in Washington.

And that is what causes the conflict.

Compared to contemporary developments, the two-story and three-story buildings along F Street are as obsolete as a Model T Ford in a mass transit system.

But like a Model T in mint condition, F Street is irreplacable, a nostalgic anacronism too good to trash, one of those things that has survived so long that it no longer merely is old but has become antique, past the point of being worthless and now invaluable.

The old stores of F Street and dozens of other 19th Century buildings downtown are an endangered species, the architectual equivalent of the snail darter. They survived in a backwater, while the growth of Washington flowed elsewhere. Now the tide of downtown development is rising around them just as the waters of the Tellico Dam threaten the darter's domain.

And the question is the same as with the snail darter: Should protection of endangered architecture block massive public improvements? Must the plans of the future defer to the remnants of the past?

The debate over how F Street's retail row can be made compatible with the rebirth of downtown Washington is scheduled to begin next month before the Joint Committee on Landmarks.

The F Street corridor is the most controversial part of a new Downtown Commercial Historic District proposed by Don't Tear It Down, the local historic preservation group.

Parts of 30 downtown blocks and 10 individual buildings would be designated as historic landmarks, protected from denigration by the same kinds of restrictions that preserve historic sections of Georgetown, Old Town Alexandria and other architectural treasures.

The preservationists spent 4,500 hours sorting through old building permits and scanning streetscapes and then nominated three related neighborhoods for landmark status.

The commercial district is the biggest, stretching across F street between 6th and 15th and up 7th Street from Pennsylvania Avenue almost to I street.

At the upper end of that 7th Street corridor is what the preservations call a "residential historic district" and everybody else calls Chinatown. Around the intersection of 14th and G Streets are nine buildings that Don't Tear it Down wants made into a historic financial district.

Within these 3 zones are 7 individual buildings nominated for historic status, including the Hecht Co. store and 2 ad joining structures at 7th and F Sts., the Homer Building on 13th Street between F and G, 3 buildings at 637, 639 and 641 Indiana Avenue, and the Calvary Baptist Chkurch at 8th and H Sts.

Outside the districts are three more newly nominated landmarks: C&P Telephone's equipment building at 730 12th St.; the only art deco building in all downtown, the Masonic Temple at 801 13th St.; and the Warner Theatre at 501 13th St.

If the Joint Committee on Landmarks -- a federal/city agency -- accepts the nominations, the seven individual buildings would be protected from alteration or demolition.

More importantly, every building in the three designated areas would become subject to the most restrictive controls in the city. Without specific authorization of the preservation police, no building could be torn down, no structure built, no remodeling undertaken. Not so much as a coat of paint could be applied to a landmark exterior without permission.

In return for declaring the 10 landmark buildings and three historic district off limits, the preservationists are offering to make the rest of downtown a free-fire zone for developers.

Sobel explained on a recent stroll across F Street that the preservationists' idea is to settle the issue of what should be torn down and what should be saved once and for all and avoid repeated wrangling over individual projects.

Don't Tear it Down specifically is promising not to raise architectural and historic roadblocks to downtown projects which are outside the three historic districts.

The preservationists are suffering, too, from lack of money. Foundation grants have not been forthcoming, and Don't Tear it Down no longer can afford to keep Sobel on the payroll as executive director.

The downtown district proposal, therefore, is more than just an effort to avoid confrontations with developers. It is the pre-emptive strike of a preservation move ment that cannot afford a long guerilla war with Washington's master builders.

Though the debate is not scheduled to begin until next month -- and is likely to be delayed -- there is plenty of reason to believe the preservationists will not get what they want.

District government officials already have told Don't Tear it Down that the proposed commercial historic district is too big, Sobel admits.

A lot of downtown is already under control of the Pennsylvania Avenue Develompment Commission. The Redevelopment Land Agency's Metro Center project and the D.C. Convention Center are going on two prime sites. The historic district takes up much of what's left.

Regardless of its roots, the 7th Street stretch contains dozens of buildings conspicuously lacking in architectural or historic merit.

Sobel stresses that individual excellence is not the point, that the fabric of 7th street ought to be saved, the mix of architectural styles and small-scale buildings that make the neighborhood more human.

It is a convincing argument. Compare the architectural stew of 7th Street to the roast beef blocks west of Connecticut on K or L or M. Look above the heads of the noontime shoppers some day and see the second-story facades with as many faces as the crowd.

Stripped of their 20th Century ticky-tacky, the stores of 7th Street would have undeniable charm, a shopping center with more character than a dozen Tysons Corners or White Flints.

The blocks of F and G streets between 11th and 16th streets are designated to become the concentrated core of downtown retail development under the District government's Living Downtown plan. Whether that can be done within the regulatory confines of a historic district is by no means certain.

The economics of preserving 7th and F Street is is another problem. Those 2-story and 3-story buildings are set on property already zoned for 10 stories. The dollars-and-cents arguments for "maxing out the cube," as builders put it, become more and more difficult to challenge as the value of downtown real estate increases.

The Mayor's Downtown Committee is supposed to issue recommendations late this year or early next about the direction downtown redevelopment ought to take. The committee, the Metropolitan Washington Board of Trade and the city planning department have not made public their views on the Don't Tear it Down historic district proposals.

Officials of all three groups (they work closely together) are urging the Joint Committee on Landmarks to go slow in considering the historic-district plans. The district government last week urged the joint committee to put off hearings on the proposal -- now scheduled for the second week of September -- until after the Downtown Committee prepares its report.

It would be a mistake to designate an historic downtown district without the input of the Mayor's Committee, but holding up the whole hearing process ought not to be necessary.

The hearings are going to take long enough as it is. Every property owner in the affected area has a right to be heard on the proposal, and the Joint Committee staff is braced for several long days.

What the preservationists and the Mayor's Committee ought to be doing now is walking the streets together, making a block-by-block, building-by-building assessment. Then they ought to sit down with their calculators, maps and zoning codes and figure out what kind if downtown historic district is economically justifiable.

The decision is too important to the future of downtown to be left to the Joint Committee on Landmarks alone.