In 1989, the private Corcoran Gallery of Art, battered by threats from Congress and worried about future federal grants, canceled an exhibition by photographer Robert Mapplethorpe that included male nudity and homosexual scenes. The controversial banning made the Washington art establishment seem philistine, intolerant and spineless.

Times and attitudes change. Now, a Washington museum is pioneering a show that celebrates gay and lesbian art and delineates its place in the history of American painting and photography. The museum is as Washington establishment as any can be: the federally funded Smithsonian Institution’s National Portrait Gallery, a museum noted more for exhibitions about presidents than shows on the cutting edge.

David C. Ward, one of the two curators, calls the show “a groundbreaking, epochal exhibition.” “Amazingly,” writes Washington Post art critic Blake Gopnik, “this is the first major museum show to tackle the topic.” Titled “Hide/Seek: Difference and Desire in American Portraiture,” the exhibition will be featured at the gallery through Feb. 13. There are no plans for it to travel anywhere else.

The show offers almost a hundred pieces, including major works by George Bellows, Thomas Eakins, Marsden Hartley, Georgia O’Keeffe, Jasper Johns, Robert Rauschenberg, Mapplethorpe and Andy Warhol. Since Ward, a National Portrait Gallery historian, and his co-curator, Jonathan Katz of the State University of New York at Buffalo, are trying to show how same-sex themes have entered the mainstream of American art, not all the artists are homosexual.

The title of the show, according to the curators, stems from the fact that sexual desire is often depicted in paintings through “the most subtle gestures, glances and codes.” This is especially true with homosexuality since it was regarded as a criminal act in the United States until late in the 20th century. As Katz puts it in the catalog, “When the desire in question is literally illegal, it is all the more fugitive, such that images of queer historical import … have passed under our contemporary perception utterly undetected.” Following the latest fashion, the curators use the word “queer” as often as “gay” to denote a homosexual.

The exhibition attempts to ferret out the hidden clues. In the 1898 painting “Salutat” (Latin for “He salutes”), Eakins portrays a 22-year-old, featherweight boxer known as Turkey Point Billy Smith as he waves to the crowd after a victory. Boxing matches in those days were all-male retreats, renowned for their brutal masculinity. But Eakins does not paint Smith in the act of fighting.

Instead, Smith is standing alone in scanty trunks that reveal most of his buttocks. The men in the crowd are ogling him much as they would a half-naked woman. This realistic 19th century sports scene is clearly homoerotic to 21st century eyes. But this aspect had long been ignored in shows and books about Eakins.

Eakins, according to the curators, was as controversial for his adulterous affairs with women as with men. He evidently wanted artists to break down all sexual inhibitions and limitations. The Pennsylvania Academy of Fine Arts dismissed him as a teacher in 1886 after he tore the loincloth off a male model in a drawing class for young women.

There is no evidence of any homosexuality by Bellows, but his works are included as honest and realistic portrayals of urban life in the early 20th century that sometimes include hints of homosexuality. In the 1915 painting “River Front No. 1,” he depicts a host of naked boys and young men swimming in a city’s river and sunning at dockside. At the far edge of the crowd, as Ward points out, “is a dandified male figure who seems to have no reason for being there, save to watch.”

Several paintings make a powerful case for the thesis that understanding a work of art often depends on knowing the biography and sexuality of the artist. Hartley’s “Painting No. 47, Berlin,” for example, is often treated as a pleasing, almost abstract work of art. But the curators point out that it has a hidden iconography.

Hartley moved to Berlin before World War I because the German capital was relatively tolerant of homosexuals. He soon fell in love with a young army lieutenant, Karl von Freyburg, who died in the first months of the war. In the painting, completed in 1915, Hartley sets down his lover’s initials, age, medal, insignia, spurs and helmet, all topped by a halo. Far from an abstraction, the painting is a memorial to the slain officer.

In a similar vein, the curators relate two works of art to each other, one by Rauschenberg, the other by Johns. The two artists lived with each other from 1953 onward for almost a decade, and the relationship helped launch the younger Johns’ career as a celebrated painter of flags, targets and numbers.

The final years and the breakup were painful, an emotion reflected in their art. In his 1959 lithograph, “Canto XIV” (from his series illustrating the cantos of Dante’s “Inferno”), Rauschenberg conjures his version of the scene in which Dante’s teacher is forced to join other sodomites in running barefoot over hell’s hot sands eternally. Rauschenberg puts down an outline of his own bare foot and includes as well little bare feet that lead to what seems like the edge of an American flag — a hint of one of Johns’ most famous paintings.

Johns’ 1961 painting, “In Memory of My Feelings — Frank O’Hara,” probably reflects the breakup as well. The title comes from a poem by a well-known gay writer who dwells on his breakup with a lover. In the view of Katz, “The image [of the painting] as a whole is a gray negative of Johns’ first painting of the American flag, the key image with which he inaugurated his relationship with Rauschenberg.” Johns originally placed a skull and the words “Dead Man” on the right side of the canvas but then changed his mind and painted them over.

For the most part, the curators reach judgments that are clear, easy to understand and well grounded in evidence. But there are times when their judgments are complex and difficult to follow. For example, they include O’Keeffe’s 1945 pastel, “Goat’s Horn With Red,” in the show, regarding it as a reflection of lesbianism.

O’Keeffe was rebelling against her husband, the photographer and gallery owner Alfred Stieglitz. He helped launch her career as a painter but also used her as a nude model, taking many photographs of her breasts, thighs and arms. Stieglitz seemed to regard women as passionate creatures instinctively offering themselves to men. “The Woman receives the World through her Womb,” he once wrote. “That is the seat of her deepest feeling. Mind comes second.”

As part of her rebellion years later, O’Keeffe moved to the Southwest and began painting animal skulls and bones that she found in the desert. Katz writes that critics have failed to notice that the dryness of the dead skulls like the goat’s horn in the pastel evaded “the generative and alluring qualities associated with female sexuality.”

Katz then takes this conclusion to another level. “Whether or not O’Keeffe had sex with other women,” he writes, “it is manifestly clear that she sought to develop a means of representing female bodies and female sexuality untouched by men — a vision, that is broadly lesbian, whether or not her sense of self followed suit.” This rationale allows the curators to include the popular O’Keeffe pastel in their show.

As the show moves toward and into the 21st century, there is little hidden about the homosexuality of the works. Yet ambiguity still rules the most interesting works. In a picture taken by photographer Annie Leibovitz in 1997, comedian Ellen DeGeneres, who had just announced she was a lesbian, is shown caught between two sexual worlds. She is cupping her breasts like a Playboy bunny, but she is also wearing boxer shorts, sporting a tough guy cigarette in the corner of her mouth and hiding her worried face behind the white makeup of a clown.

The exhibition had an unusual beginning. In 2006, Ward, a scholar of American history with no background in gay studies, curated a show on Walt Whitman for the National Portrait Gallery. He included a photograph of Whitman with his lover Peter Doyle and identified Doyle as such in the label.

Later, at a symposium on Whitman, Katz, a specialist in gay studies, introduced himself and surprised Ward by telling him that Doyle had never before been identified as Whitman’s lover in a major museum exhibition. Conversations between Ward and Katz led to their proposal for an exhibition that would survey the role of homosexuality in American art.