Why is おもだった in past-tense?

Question

In

すでにおもだった犯罪者の名前は書き尽くし徐々に悪人のレベルも下げてる

Here おも means "important", so おもだった犯罪者の名前 means "important criminal's names". But why is もだった in the past tense? Instead of:

すでにおもな犯罪者の名前は書き尽くし徐々に悪人のレベルも下げてる

Answer 1

主だった is a fixed 連体詞 that must be memorized as-is. This だった is not the past form of だ but the past form of 立つ. You can write it also as 主立った. (For clarification, 連体詞 refers to lexicalized words like あらゆる, 大した and 色んな. They never conjugate, and they must be memorized as-is.)

Some dictionaries, including デジタル大辞泉, acknowledge the existence of the verb 主立つ. However, practically speaking, 主だった is almost always used in this form, and very rarely the adverbial form 主だって appears. 明鏡国語辞典 explains 主だった as a lexicalized 連体詞 and does not list 主だつ as a distinct verb.

If you're still wondering why it ends with 立った, this answer may be related. Still, the correct approach for this word would be to memorize it in this specific form.