On a single day last fall — Oct. 10, to be exact — more than two dozen artists gathered in a New York City studio to paint a 76-year-old woman. She wore a black robe and shell-shaped earrings and was seated on a floral-printed chair set against a plain, sky-blue backdrop.

The artists in question were all members of The Painting Group, founded in 1958 as a weekly class whose fees paid the rent on caricaturist David Levine’s studio. Today, the group includes a father

and son, a stockbroker, the daughter of a prominent writer, a master of American portraiture and a former psychiatrist. Meanwhile, the group’s septuagenarian subject — former Supreme Court Associate Justice Sandra Day O’Connor — was once one of the most powerful women in the country.

The session, which lasted six hours, ultimately yielded 25 portraits of O’Connor, all of which are included in a single-room exhibit opening Friday at the National Portrait Gallery. Excerpts from a documentary that was filmed during the sitting also will be on view in the exhibit.

As the court’s one-time key swing-vote, O’Connor, who turned 77 on Monday, always has possessed a certain mystique, which makes her fertile ground for interpretation.

“She knows how to be controlled,” said the 80-year-old Levine, a frequent contributor to The New Yorker whose illustrations have adorned the covers of Time and Newsweek. “She can’t be read.”

Accordingly, the works present a stunning study in the subjective nature of perspective — one woman really can be one man’s elegant dowager, another’s power gal and still another’s age-ravaged senior citizen.

Take the interpretations of the two best- known artists in the show — those of the famous portraitist Aaron Shikler, whose strikingly modern image of the late Sen. Mike Mansfield (D-Mont.) hangs in the Capitol, and Levine. The two men, who teach the group’s weekly Wednesday evening class, offer startlingly different views of O’Connor.

Shikler turns out a supremely detailed pastel rendering of a diminutive head set atop shoulders that consist of little more than a few lines. The dominant non-facial feature here is O’Connor’s trademark jabot, the lacy ruffle she wore on the outside of her judge’s robe. Meanwhile, Levine’s wildly rendered white watercolor with its splotchy pink blemishes transports O’Connor to a dimension somewhere between that inhabited by Daumier’s rabble and a blowzy madam in Toulouse-Lautrec’s Montmartre. (Levine’s son, Matthew, offers a quietly contemplative oil painting of O’Connor more in line with the work of Shikler, whom he lists as a teacher, than his father’s.)

That said, the vast majority of these artists play it safe — but even among the traditional portraiture there is wide variation.

On one wall of the exhibit hang five representational Gilbert Stuart-style, bust-length portraits of roughly the same size and scale — though the definition, coloring and detail of O’Connor’s jabot vary slightly from picture to picture, which can mean dramatically different effects. For instance, Hannah Achtenberg Kinn gives us a grandmotherly, almost Barbara Bush-esque O’Connor with white cropped hair and a matronly smile. Irene Vitale, on the other hand, manages to imbue a blonder O’Connor with a sort of Helen Mirren “woman of a certain age” sexiness.

Other portraits invoke O’Connor’s Arizona heritage. There’s a fetching little abstract watercolor by Jane Schube, where a seated O’Connor, a former Grand Canyon State appeals court judge and state Senate Majority Leader, appears in patches of color from a Southwestern palette — bright blues, browns and golds. Adam Van Doren’s caricature looks like it’s been the victim of a rolling pin covered with rusty-brown dirt. Perhaps the show’s most realistic representation of O’Connor — an oil canvas by Gil Eisner — plops her head against a sky of purple, blue and rose, giving O’Connor, who grew up on a cattle ranch, a whiff of Western-style manifest destiny.

“There is a huge personality, a huge human being with great feeling,” Levine said of O’Connor.

A few pieces also show her softer side. Chief among these is an oil by Pamela Talese, daughter of the writer Gay Talese. Her wide-eyed O’Connor’s very feminine (and rather youthful) head sits on a pair of robed, exaggeratedly wide shoulders — more suited to a linebacker, which makes for a nifty juxtaposition between her gender and the very male enclave of the “brethren” O’Connor entered when President Ronald Reagan appointed her the court’s first female associate justice in 1981.

And some portraits are just plain weird.

Laura Duggan’s creepy folk art painting of O’Connor imbues her with an otherworldly quality. The colors are flat and unmodulated and the greenish-blue eyes seem to glow with a devilish zeal. It’s O’Connor as actress Christina Ricci à la “The Addams Family.”

Then there’s Emily Epstein Vines’ tiny O’Connor sculpture — the only one in the show — which sits on a white pedestal not unlike the columns of the Supreme Court itself. The tacky George Washington-invoking piece is about as campy as you can get and even includes a piece of real lace tacked to O’Connor’s bosom. It’s the sort of tchotchke one might find at a street-side souvenir stand amid the FBI T-shirts and presidential magnets.

On second thought, maybe it’s perfect for Washington, D.C.

“Portraits of Sandra Day O’Connor” is on view from March 30 to Oct. 8 at the National Portrait Gallery, located at Ninth and G streets Northwest. For more information, visit www.npg.si.edu.