Martin Luther King Jr.’s influence on American politics and his views about policy issues are a perennial topic of discussion around the time of his January 15 birthday and the Martin Luther King Jr. Day federal holiday. However, the civil-rights leader’s personal political party affiliation remains a mystery.

His niece Alveda King, an Evangelical supporter of President Donald Trump, has argued that her uncle was a Republican, like his father Martin Luther King, Sr., who was also a Baptist minister. That idea has been repeated often, but videos that claim to show that Martin Luther King, Jr. is Republican have been proven not to do so. King’s son Martin Luther King III said in 2008 that it’s “disingenuous” to insist he was when there is no evidence of him casting a Republican vote. “It is even more outrageous to suggest that he would support the Republican Party of today,” the younger King added, “which has spent so much time and effort trying to suppress African American votes in Florida and many other states.”

The idea that King would have been a registered Republican is not far-fetched, given the party’s history and its position in national politics in the 1950s, but scholars and those who knew him best say they can’t imagine that he would have supported Republican presidential candidates in the 1960s. In fact, King himself said he voted for Democrat Lyndon B. Johnson for President in 1964.

“I know of no one who has verified MLKJ’s party registration,” says Clayborne Carson, editor of King’s autobiography and Professor of History and Founding Director of The Martin Luther King, Jr., Research and Education Institute at Stanford University. “[He] may have been registered as a Republican and voted Democratic [in national elections].”