Can を used with だ/です? 君沢さんを犯人だと思い込む

Question

I came across the following passage in a mystery manga. The main character is explaining that the as of yet unknown criminal intentionally made the listener think that Kimisawa was the criminal (though she is not).

おそらく奴はお前が君沢さんを犯人だと思い込むのを計算に入れて、その「香水」の匂いをかがせたんだ！
It's likely that that guy (奴) incorporated it into his plans that you'd be under the impression that the criminal was Kimisawa. He made you smell that perfume on purpose!

Unless I am mistaken, it seems to me that the subject/complement of だ is denoted with を which I thought was unacceptable. Could someone please explain when and if this is allowable?

Answer 1

AをBだ in isolation makes little sense (although there are minor exceptions). This ～を～だと is a common pattern which appears along with various verbs for assuming, regarding, etc.

• AをBだと見なす　to regard A as B
• AをBだと考える　to consider A B
• AをBだと仮定する　to assume A is B
• AをBだとする　to suppose A as B
• AをBだと思う　to think of A as B
• AをBだと勘違いする　to mistake A as B
• AをBだと思い込む　to make a wrong assumption that A is B
• AをBだと定義する　to define A as B
• AをBだと受け取る　to take A as B
• AをBだと解釈する　to interpret A as B

So Bだと is like as B, and these verbs can take both を and だと at the same time.

EDIT: だ before を can be omitted in many cases. 君沢さんを犯人と思い込む (without だ) and 君沢さんが犯人だと思い込む are equally correct. For details, see Difference between だと vs と before 認める and grammatical and semantic difference of "嘘と思う" and "嘘だと思う"

Answer 2

I don't know if it's allowable per say, but it's clear that お前が君沢さんを犯人だ is meant to be a thought in it's own right, which is what (that guy) took into consideration.

I would guess what it's supposed to mean is

that (that guy) assumed you would assume Kimazawa is the criminal, and so he had you? smell that perfume.

As for correctness, I was able to find another example on Linguee that uses を犯人だ

ワシントン弁護士は周到にもハシモトが一切現場に行ってないという警察側の証人まで用意して、もし証拠もなく私を犯人だ、と証言したらすぐ彼らを名誉毀損で訴える準備をしていた。

And it kind of makes sense. The subject is attributing object being some noun.