A group of preservationists threw one of Mayor Marion Barry's unkept 1978 campaign promises back at him yesterday as it launched a last-ditch effort to save Rhodes Tavern, the oldest commercial building in downtown Washington.

The 180-year-old structure at 15th and F streets NW is in a block where developer Oliver T. Carr plans to erect a $70-million commercial and office development that has gained supported from the Barry administration. The plans call for relocating or demolishing the tavern building, now occupied by four small ground-floor stores.

Joseph N. Grano Jr., a lawyer and cofounder of the Citizens Committee to Save Historic Rhodes Tavern, produced a typewritten 1978 campaign statement in which Barry said that if elected mayor "I will be continuing my support of the preservation" of the tavern and two nearby landmarks. Barry "reneged on his campaign promise . . . he is no longer committee," Grano declared at a news conference.

Barry, speaking later through his press secretary, did not dispute the authenticity of his 1978 statement, but insisted that "I promised to try (to save the tavern) and I did try . . . You can't win 'em all."

Grano and another cofounder of the committee, Jerrie Linder, said they called yesterday's news conference inside the forlorn-looking tavern in the hope of mobilizing public support for keeping and restoring the structure on its present site across from the U.S. Treasury.

Its fate may be decided by a hearing next Monday scheduled by D.C. housing director Robert L. Moore, the mayor's representative in executing the city's historic preservation law. The hearing will begin at 10 a.m. in Room 970 at 1325 G St. NW.

The tavern building is the oldest commercial structure in downtown Washington. Those who want to save it have noted that it is only structure still standing that has been along the route of every president's inaugural parade, from Jefferson's in 1801 to Carter's in 1977.

It served a number of civic purposes, from polling place to courthouse, and was the first home of the National Press Club.

At the time Barry made his 1978 statement on its preservation, Carr already had announced plans for fully redeveloping the rest of the downtown block occupied by Garfinckel's department store. The plans included demolition of Rhodes Tavern and the nearby Keith-Albee and National Metropolitan Bank buildings regarded as outstanding examples of Beaux Arts architecture.

Barry hoped in 1978 to gain votes from preservationists, some of whom believed then-mayor Walter E. Washington was not doing enough to save the structures.

Earlier this year, after Carr began and then halted the demolition of Keith-Albee, officials of the Barry administration headed by James O. Gibson, an assistant city administrator, negotiated a compromise with the developer that was endorsed by the mayor.

It called for moving Rhodes to another location if possible, or demolishing it, and incorporating the facades of the other two buildings in the new Carr development.

James W. Symington, a board member of another group called the Committee to Preserve Rhodes Tavern, said his organization accepted the compromise and hopes a new site can be found for the tavern. Grano said at yesterday's news conference that Rhodes would lose all historic significance if moved elsewhere.