The arc of a mistake is long, and it now stretches from the Oval Office over to the Mall.

An error has been etched in marble on the grand Martin Luther King Jr. memorial that was to be dedicated Sunday, on the 48th anniversary of King’s “I Have a Dream” speech. Some of King’s speeches and writings have been inscribed in the memorial. But one of the sayings on the wall by the Tidal Basin is incorrect — or incomplete — in its attribution.

“The arc of the moral universe is long, but it bends toward justice.”

According to David Remnick’s biography of Obama, that is the president’s “favorite quotation.” Obama brought the idea back into present-day parlance and even had it sewn into the rug in the Oval Office when he redecorated last year. But as I wrote on this page last September, King is not the source of that quote.

The president should correct the record on words he cherishes, which are mistakenly and commonly cited as King’s.

Theodore Parker, a long-gone Bostonian abolitionist and Unitarian minister, is the true author. The charismatic Parker died at age 49 in 1860, just before the Civil War.

In King’s heyday, the civil rights leader and Southern Baptist preacher often joined Parker’s “arc” quotation with his own refrains of “We shall overcome” or “How long? Not long.” On the gleaming curving wall of the King memorial, the “arc” quotation is given simply as King’s, spoken in 1968 in the District of Columbia. The lines are presented with more than a dozen other lyrical passages of his oratory and the 1963 “Letter From a Birmingham Jail.” An excerpt from King’s Nobel Peace Prize acceptance speech in 1964 is also part of the massive memorial, which faces the Jefferson Memorial across the Tidal Basin in a defiant historical dialogue.

King saw Parker as a kindred spirit on the front lines of America’s first civil rights movement. During the bitterly divided 1850s, Parker belonged to a network of abolitionist Quakers, Unitarians, Transcendentalists and Underground Railroad organizers. This vanguard of whites and blacks embraced nonviolent civil disobedience, the philosophy of social change that King embraced in his own time and place.

King honored Parker’s original thought by breathing new life and meaning into his 19th-century words. There is no question of plagiarism; King made no secret of the source. As clergy, they also shared a religious basis for their social ideals.

King forcefully revived Parker’s “arc” quotation several times in the 1960s. Years later, Obama loved the line so much that he chose it as one of five for the Oval Office rug — without reference to Parker.

Let’s not allow the same omission in a place of remembrance that will be visited by countless Americans in the decades to come.

The Parker-King connection across generations and color lines should be celebrated as an example of how an unfinished life work can be carried on.

Parker, an antebellum champion of social progress, is ironically neglected in the national narrative he helped to write. His family went back to the Revolutionary period’s Minutemen at Lexington, Mass., where his grandfather commanded troops. Born to the white, privileged class of a new republic, he believed in expanding the nation’s rights and freedoms to live up to its words on paper. He would have understood what King meant by the “fierce urgency of now.”

The night before he was assassinated in Memphis in April 1968, King said: “I’ve seen the Promised Land. I may not get there with you.” Parker died in 1860 before seeing the dream of Emancipation come true.

For the record, Parker said in 1853: “I do not pretend to understand the moral universe; the arc is a long one . . . But from what I see I am sure it bends toward justice.”

Perhaps someday soon history’s arc may bend and give Parker a little justice of his own.

Jamie Stiehm, a journalist, is writing a book on the life of Lucretia Mott, a 19th-century Quaker abolitionist and women’s rights leader.