WASHINGTON — The idea for “The Sweat of Their Face,” a coming exhibition at the Smithsonian’s National Portrait Gallery, came from a plumber.

When Dorothy Moss, the museum’s curator of painting and sculpture, was researching her graduate dissertation, she found news accounts from 1897 about a plumber who visited the Metropolitan Museum of Art on a break but was asked to leave because he was wearing overalls.

Thinking of the plumber, Ms. Moss became interested in one of the central questions of museums: Whom are they for?

“The Sweat of Their Face,” which she curated with the museum’s senior historian, David C. Ward, tries to address that question. Consisting of more than 75 portrayals of American laborers from the 18th century through today, the gallery will open in November as part of the museum’s 50th anniversary celebration.