Antonyms for the phrase "cast doubt upon" include "validate," "support," "endorse," "confirm," and "uphold." When one attempts to cast doubt upon something, they are essentially questioning the legitimacy or truthfulness of the underlying subject or statement. However, using the aforementioned antonyms in place of "cast doubt upon" suggests that the subject or statement has been validated, supported, or confirmed to be true. Instead of questioning the legitimacy of a claim, using these antonyms suggest that the claim is sound and trustworthy, providing evidence or testimony to support its validity. Thus, using these antonyms can help establish a more confident and positive tone in communication.