The phrase "declare against" typically implies that someone is publicly announcing their opposition or disapproval of something. There are many synonyms that can be used to convey a similar meaning. One common option is "denounce," which suggests a strong, explicit condemnation of something. "Oppose" and "object to" are also useful synonyms, which both imply a stance against something without necessarily using such forceful language. "Refute" can be another option, specifically if someone is attempting to disprove an argument or statement. Overall, there are many synonyms that can be used effectively to convey the idea of declaring against something, depending on the level of intensity and specificity desired.