Can the attention of a fickle museum-going public be turned toward the Hirshhorn Museum and Sculpture Garden, now that the massive Impressionism and "Treasure Houses" shows have left the National Gallery of Art?

"Robert Arneson: A Retrospective," showing April 30 to July 6 at the Hirshhorn, has at least one thing in common with the previous two attention-getters: Its works have never before been exhibited together.

Among those works will be the California sculptor's "Portrait of George." The subject is slain San Francisco mayor George Moscone, whose death in 1978 is part of a saga that has been commemorated, chronicled and, in some cases, mined in almost every discipline except dance. "Execution of Justice," a play recently performed at Arena Stage, deals with the circumstances surrounding Moscone, gay city supervisor Harvey Milk, and their assassin, Dan White, the former city supervisor who later committed suicide. "The Life and Times of Harvey Milk," a film that explored Milk's political career and the city where he lived and worked, was released in 1984.

"Portrait of George" was requested by the San Francisco Art Commission for the city's Moscone Convention Center. The sculpture cost the commission $37,000. But after the new convention center was completed and the sculpture installed, the commission had second thoughts. Reportedly, the sculpture's pedestal, with its references to the assassination, disturbed Moscone's widow Gena and San Francisco Mayor Dianne Feinstein. The sculpture was subsequently removed, and Arneson returned the advance he had received. It was later bought by Foster and Monique Goldstrom, art collectors from Oakland and Dallas, who have lent it to the show.

In addition to portraying Moscone, "Portrait" vividly evokes the events surrounding Moscone's death. Five bulletlike holes pierce the sculpture, blood flowing out of each hole. Grouped around the sculpture are words and images, including a Hostess Twinkie, a reference to the now-famous "Twinkie defense" used by White at his trial.

The sculpture has been lent for the Washington leg of the exhibition, which opened in February at the Des Moines Art Center. Hirshhorn public information officer Sidney Lawrence said he did not know whether the Moscone sculpture will continue with the exhibition when it travels to Portland, Ore., on Nov. 6. George and Martha Return

By the terms of a joint custody agreement governing their location, a very famous American couple returns to Washington next month for a three-year stay. The Gilbert Stuart portraits of George and Martha Washington will return to the National Portrait Gallery, one of the owners, from the Boston Museum of Fine Arts, the other owner. A 1980 agreement provided for the joint purchase of the portraits by the institutions after a much publicized, yearlong legal battle. The Stuart portraits alternately spend three years with each owner. The portraits are to be installed in the gallery's second-floor rotunda. Sotheby's Sums

Last year was a very good one for Sotheby's auction house, which reported worldwide sales of $678.8 million. This represents a 15 percent increase in sales over 1984. Christie's, an international competitor, does not release figures for the calendar year, but rather computes its sales based on a September-July auction season; its sales for September '84 through July '85 were $475.9 million. Canceled Concert

The Washington Chamber Orchestra, limping by financially, was forced to cancel tonight's scheduled concert at the Kennedy Center. Orchestra board member Paul Teare had warned concertgoers two months ago that lessening corporate and government support threatened to curtail the orchestra's performance season. The concert was to have featured violinist George Marsh and cellist Steven Thomas, winners of last year's Bach International Competition. NEA Grants

The National Endowment for the Arts has announced grants to 107 presenting organizations for the 1986-87 performance season. In Washington, District Curators received $23,500, some of which is destined to finance a production of "the Knee Plays" segment from Robert Wilson's "The CIVIL warS." The Kennedy Center received $30,000 for a program featuring theater companies from across the country, a performance artist series and a musical theater series. The Washington Performing Arts Society got the largest amount in Washington -- $50,000 -- to develop its audience.