Tom Buchanan believes in a racist philosophy based on the superiority of whites, especially northern European whites. Early on in the novel, he tells Nick Carraway about a book he has read called "The Rise of the Colored Empires" by "this man" Goddard. Even though both Daisy and Jordan interrupt him, clearly trying to steer the subject into more comfortable channels, Tom persists in explaining Goddard's theory that the northern Europeans--the "Nordics"-- had developed art and science: "all the things that go to make civilization."
As Tom puts it, "if we don't look out, the white race will be --will be entirely submerged. It's all scientific stuff; it's been proved."
When Daisy attempts to ridicule him, saying "He reads deep books with long words in them," Tom interrupts to repeat: "This fellow has worked the whole thing out. It's up to us, who are the dominant race, to watch out or these other races will have control of things."
Tom says that "civilization is going to pieces," and refers to himself as pessimist.
He identifies Nick, Jordan, and himself as Nordics. "There was something pathetic in his concentration, as if his complacency ... was not enough for him anymore," Nick notes. If Tom was disillusioned, this philosophy provided him with an answer for why the world seemed to be going downhill.
Interestingly, there is "an infinitesimal hesitation" before Tom includes Daisy into the rarefied company of Nordics that Jordan and Nick occupy without any second thoughts. This indicates that Tom could be blaming some of his marriage problems, as he is having an affair, on a vague sense of Daisy's racial impurity.
While Daisy repeatedly makes fun of Tom's racist philosophy, it affects how Tom deals with Gatsby. Tom never accepts Gatsby as a Nordic, constantly raising questions about where he came from and who he is. It's also important to note that this kind of racist thinking was much less reviled in the 1920s than today and that a Lothrop Stoddard had written a book called "The Rising Tide of Color: The Threat Against White World" Supremacy" that argued for a eugenic separation of the races.