During the early Cold War, the Red Scare in the United States led to widespread accusations of disloyalty. As part of this activity, Senator McCarran headed the Senate Internal Security Subcommittee which called Phillips to testify. Phillips worked not only with Robert Oppenheimer on nuclear physics and the Oppenheimer–Phillips process, but also worked with the Teacher's Union. When questioned about whether she was involved with the Communist party, Phillips chose to neither confirm or deny, but to simply state that her lineage goes back just as far as any other American.