Final answer:
President Carter referenced the overwhelming Congress vote to support the claim of the boycott's popularity, implying that popular public opinion is reflected through their representatives' support.
Step-by-step explanation:
When President Carter claimed that the decision to boycott the Moscow Olympics was widely popular, he referenced the support from the United States Congress as his evidence. The relevant passage from his speech that supports this claim is: "The Congress has voted overwhelmingly, almost unanimously, which is a very rare thing, that we will not go."
This statement pertains to Congress’s support for the boycott and suggests that Carter is using this fact to imply that, because the representatives of the American people are in favor, the decision must indeed be popular with the public.