WASHINGTON—It was a text-message debate that led Scott Sandage and Jonathan White to discover a vital American artifact last weekend: a long-forgotten letter showing how Frederick Douglass really felt about a statue of Abraham Lincoln and a slave.

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.