According to the usual authorities the particle ecce is construed with the accusative in pre-classical Latin, but with the nominative in classical and post-classical Latin. Thus, Lewis and Short: “(constr. class. with nom. or clause, and ante-class. also with acc.)”, with copious references to writers like Terrence and Plautus saying things like ecce me, or ecce hominem.
Greek distinguishes (perhaps artificially?) ἰδοῦ, the aorist middle imperative of the verb “to see”, obviously taking a direct object in the accusative case, and the particle/adverb ἰδού, “behold!”, which does not take a direct object. So we have famously in John 19:5: ἰδοὺ ὁ ἄνθρωπος, Vulgate ecce homo.
The Latin ecce is clearly not a verb, but the early poets seem to treat it like a verb, perhaps by analogy to Greek ἰδοῦ / ἰδού, while the classical writers revert (as it seems) to the authentic Latin usage as an adverb. Or is something else happening?