When it comes to synonyms for the phrase "backs up", there are several options to choose from. Some of the most commonly used options include "supports", "upholds", "backs", "confirms" and "endorses". These synonyms can be used interchangeably depending on the context in which they are being used. For instance, if you want to emphasize that someone agrees with your statement or position, you can use the word "endorses". Similarly, you can use the word "upholds" when referring to a code of ethics or set of rules. In essence, using synonyms for "backs up" enables one to vary their expression and widen the scope of communication, making it more engaging and informative.