Group Material is a New York artists' collective that accentuates the populist. ''Send us something -- anything'' is its message of the moment.

When ''Messages to Washington,'' the group's show at the Washington Project for the Arts, opens tomorrow in a monumental room painted stately federalist blue, the material will be thin indeed. The reason, as members of the collective readily admit, is that the pubic response to their ''open call'' for pictures, objects, writings and taped messages has been decidedly weak.

''I don't know whether it's our country,'' says photomontage artist and legal secretary Mundy McLaughlin, a charter Group Materialist, ''or maybe we used the wrong strategy. I think we should just think of this as an experiment.''

It was a few months ago that the group of four part-time artists -- which gets by on private, state and federal grants -- spent $1,100 placing ads insmall-town newspapers al over America, from the Tonawanda News in Upstate New York to the Siskiyou News in Yreka, Calif., exhorting the populace to ''TELL YOUR OVERNMENT WHAT YOU THINK.''

The result on exhibit, filled out at the last minute with contributions from such established painters and sculptors as Howard Finster and Christy Rupp as well as photographs from newspapers and magazines, is an artfully arranged hodgepodge of American detritus. There are political posters, scribbled letters, broken toys, a torn sneaker -- a veritable trash heap of social portent.

''NO MORE FORCED EVICTIONS FOR THE POOR,'' declares a Magic-Markered signboard by Dorothy Kohn, an elderly tenants' right activist-cum-graffiti artist in New York, plkaced on a pedestal of indigenous Washington rubble. ''I'M BROKE, MR. REAGAN,'' cries an anonymous midwestern farmer; actually, he has plowed the message across a fallow field and an aerial photographer has preserved it for full-color posterity.

Group Material received more letters than anyting else, about 20 in all, and these are displayed as orginals or photostatic blowups. They come from people apparently disturbed about something, or just plain disturbed. A disabled veteran n Oregon, for instance, writes to assail everything from the lassitude of his insurance company to the disgraceful condition of Highway 101.

The show's appeal is more verbal than visual, maintly because Group Material had more words than images to work with. It necessarily suffers from lack of curatorial discretion -- everything the group got it used -- and also from the nonparticipation of folks out there. Group Material seems entirely sincere in its emergency call for more messages. But maybe Americans these days don't have much they want to say. The show, at 400 Seventh St. NW, closes Oct. 12.