It's difficult sometimes, or impossible, to draw a distinction between what politicians say publicly for the benefit of their constituents and what they actually believe. But Strom Thurmond seems to have truly been deeply racist: "[A]fter listening one day in 1957 to [Thurmond] deliver, in a dispassionate tone, a long, dogmatic discourse on the irremediable inferiority of the Negro race, Olin Johnston, ardent racist himself, was moved to comment: 'Strom really believes that stuff!'"

This is from the third volume of Robert Caro's definitive biography of Lyndon Johnson, Master of the Senate, p. 866; the ultimate source seems to be the author's interview with one Robert Barr of U.S. News & World Report, a reporter who worked in the Senate. There are plenty of anecdotes about Strom's racist positions and what a standard-bearer for segregation he was, but this anecdote strikes me as more relevant to OP's question than that stuff.