Edit: So I guess a more accurate way to put that particular aspect of Rushmore’s history would be, instead of “it was funded in part by the KKK,” one could say “the project was spearheaded by an avid supporter of the KKK.” Or even just “a white supremacist.”

I did some searching, here’s what I can find on it from a snopes writeup:

https://www.snopes.com/news/2020/07/29/kkk-mount-rushmore/

The man behind the mount, Borglum, had an old relationship with the KKK, preceding his time as the designer and sculptor of Mount Rushmore. In 1914, the United Daughters of the Confederacy — an organization known today for stopping the removal of Confederate monuments — approached him to create a “shrine to the South” on Georgia’s Stone Mountain, about a thousand miles south from where Mount Rushmore would be. In 1915, the KKK would be reborn (it had faded during the Reconstruction Era following the Civil War) in a ceremony on Stone Mountain.

Borglum was an “avid and influential supporter” of the KKK, Taliaferro wrote in “Great White Fathers,” even though there was no proof that he was a card-carrying member of the organization. He was involved in their politics, attended rallies, served on committees, and saw them as a source of funds for his work on Stone Mountain. He was a white supremacist who said, “I would not trust an Indian, off-hand, 9 out of 10,” and wrote, “All immigrants are undesirable,” even though his father was a Danish immigrant. He also took great pride in his Norse heritage, according to his writings.

The KKK did financially back the Stone Mountain project, even though Borglum tried to obscure its involvement. But infighting within the Klan by the mid-1920s, as well as stalled fundraising for the monument, led to Borglum leaving the project. He was approached by a historian to take on the Mount Rushmore project in South Dakota, enraging his backers on Stone Mountain. By 1927, he began carving Mount Rushmore, devoting the last 14 years of his life to the project that was finished by his son.

The KKK does not appear to have been behind any funding for Mount Rushmore. According to Deloria, Borglum received mostly federal funding for Rushmore, and he had left too much bad blood behind in Georgia to receive further funding. Taliaferro described how Borglum and the Mount Rushmore committee struggled to find funds for Rushmore for a few years. They scraped together finances from magnates and a senator, and by 1929 received federal funding. Out of the total expenditure of $989,000, the government had contributed $836,000, according to “Great White Fathers.”

Even though this meme highlighted key elements of Mount Rushmore’s darkest history, some of its facts were incorrect or pulled out of necessary context. While the man behind Mount Rushmore was very closely aligned with the KKK, evidence suggested that the organization itself did not fund the monument’s creation. But the monument remained tied to a racist past, highlighting figureheads who were slave owners and despised by Native Americans, and built on land that was indeed stolen by the U.S. government.

Hmm. Would be nice to have a source to go on in either direction, some kind of details on its funding.