Messrs. Sandage and White are history professors who have been on opposite sides of a dispute over the Emancipation Memorial near the U.S. Capitol, which depicts Lincoln in the act of freeing a kneeling Black man.

Mr. White, who teaches at Christopher Newport University in Virginia, wrote in a newspaper that the statue should be preserved, even while conceding in passing that Douglass disliked the design.

Mr. Sandage, a professor at Carnegie Mellon University in Pennsylvania, considered the statue “visually unredeemable” because of its depiction of a Black man kneeling in a subservient position to Lincoln.

Both men sit on the board of the Abraham Lincoln Institute and had been debating whether the statue should remain or come down.

And so on the last Friday evening in June, sitting on the couch with his wife watching “Gilmore Girls,” Mr. White was texting back and forth with Mr. Sandage, pondering the alleged distaste for the statue by Douglass, who had dedicated it with a famous address in 1876.

The account of Douglass criticizing the statue at its unveiling came from a 1916 book that included the recollection of activist John W. Cromwell, who was in attendance.

Mr. White pointed out the account was secondhand from three decades later, and could be apocryphal. Mr. Sandage had thought Cromwell’s account had been corroborated and cited it in his own work in the 1990s. He went searching for a corroborating account.

Last Saturday morning, Mr. Sandage started searching Douglass’s name and the word “knee” in digitized newspaper archives at Newspapers.com. He found no corroborating accounts of the remark, but something better: published blurbs headlined “Frederick Douglass says” that referred to an 1876 letter from Douglass criticizing the monument.

After 20 minutes, and narrowing the search using Douglass’s flashiest adjective (“couchant”), Mr. Sandage uncovered Douglass’s letter itself.

[A letter to the editor of the National Republican newspaper in Washington from Frederick Douglass in 1876.
PHOTO: LIBRARY OF CONGRESS]

Five days after the unveiling, in a letter to the editor of the National Republican newspaper in Washington, Douglass had critiqued the statue’s design and suggested how more dignified depictions of free Black people would improve the park.

“The negro here, though rising, is still on his knees and nude,” Douglass wrote. “What I want to see before I die is a monument representing the negro, not couchant on his knees like a four-footed animal, but erect on his feet like a man.”

Mr. Sandage said he didn’t at first realize the importance of his discovery, but alerted Mr. White and texted an image of the letter to David Blight, a Douglass biographer and history professor at Yale University.

Mr. Blight was “practically giddy,” Mr. Sandage said.

Mr. Blight in turn emailed Richard Fox, a Lincoln scholar at the University of Southern California, who hadn’t seen the letter either.

“This all happened on Saturday morning,” Mr. Fox said. “None of us knew until three days ago that there was any evidence in Douglass’s entire life that he had actually said these things, and then there it was.”

Mr. White and Mr. Sandage weren’t done. Their searches also uncovered an obituary for Charlotte Scott, the former slave whose $5 donation had kicked off the fundraising to pay for the monument on the day of Lincoln’s death.

The statue was paid for by donations from former slaves, including Black veterans of the Union Army, but the design was selected by the Western Sanitary Commission, a St. Louis charity run by white people, according to the National Park Service.

The commission selected the design by Thomas Ball, an American sculptor living in Trieste, Italy, after years of appeals failed to raise sufficient funds for a larger and more complex monument, historians said.

Messrs. White and Sandage also found a reference in the Washington Bee, a Black newspaper in the city, to “the Charlotte Scott Emancipation statue in Lincoln Park.”

Just like that, a document apparently unknown to Douglass’s biographers and not found in the orator’s papers at the Library of Congress had landed squarely in the middle of the debate that has swept the nation and the neighborhood around Lincoln Park where the statue stands.

Amid the Black Lives Matter Movement and the protests following the killing of George Floyd, momentum is gathering to remove or alter statues like the Emancipation Memorial, following successful calls to take down monuments of Confederate generals.

In Washington, a candidate for District Council, Marcus Goodwin, has gathered roughly 7,000 signatures on a petition to either remove or alter the Lincoln statue. Eleanor Holmes Norton, Washington, D.C.’s nonvoting representative in Congress, has said she would introduce legislation to move the statue to a museum. And in Boston, a panel voted unanimously on Tuesday to take down a replica of the Emancipation statue.

Mr. Goodwin has said that concerns about the statue could be addressed by adding more Black figures to the statue that are in standing positions, including contemporaries of Lincoln like Douglass. It is a compromise that the newly discovered Douglass letter seems to anticipate.

Still others believe the existing monument should be moved, including Kirk Savage of the University of Pittsburgh, whose work includes “Standing Soldiers, Kneeling Slaves,” a history of monuments erected after the Civil War.

“It is a distorting image,” Mr. Savage said. “It’s a white savior narrative that puts Lincoln in the position of a kind of saint, working a miracle cure on the enslaved population.”

If new additions to the memorial are done right, Mr. Sandage said, “the original statue would become an artifact and the new groupings around it would become the focus.”

Mr. White said that “people of good will are on both sides” of the issue. “If people had listened to [Douglass] it might have resolved it 144 years ago,” he said.

As for their discovery, Mr. Sandage credited the activists, whose demonstrations at the park had led to his debate with Mr. White.

“That’s how historians work,” he said. “We argue with each other and then go look again.”

Messrs. White and Sandage said the find helped them reach an agreement, which they proposed this week in an article for Smithsonian Magazine. Citing Douglass’s words, they argued that “no one monument could be made to tell the whole truth of any subject which it might be designed to illustrate.”

The historians suggest adding more statues—of Douglass and of Scott —and better explaining the story of Archer Alexander, who was the model for the slave figure. Mr. Alexander was the last man arrested under the Fugitive Slave Act, the Park Service says.

“If the statue is to stand there any longer, it should no longer stand alone,” they wrote. “Who would be more deserving of honor with an additional statue than the freedwoman who conceived of the monument?”