WARNING: The following article contains spoilers for Batman '89 #3, on sale now from DC Comics.
Harvey Dent didn't become Gotham City's District Attorney because he was slouch. He's a highly intelligent man, perhaps even more so in Tim Burton's version of Gotham City than in any other universe. This version of Harvey was a keen politician, able to work both sides of the city in his favor because of his genuine desire to change things for the better. But behind the scenes, Harvey was also a decent detective, rapidly piecing together facts about Batman in an effort to figure out who was behind the mask, and Batman '89 #3 (by Sam Hamm, Joe Quinones, Leonardo Ito, and Clayton Cowles) shows just how close Harvey was to uncovering Gotham's greatest secret.
Harvey had already been on the trail of Batman, using existing evidence to deduce where he gets his equipment. A quick examination of a leftover Batarang and its heat-seeking technology convinced Harvey that whoever made these things did not require a fortune because they already had one. This significantly narrowed down the list of people who might be Batman. But another clue came when Bruce slipped up and mentioned a girl who had recently been orphaned when her name had not been released to the public, something only someone who was there could know.
Despite all this, Harvey never got a chance to confront Bruce Wayne about it. In an effort to rescue a civilian from a fire, Dent was trapped in the blaze, and suffered hallucinations indicating that his mind was going in a slightly different direction. By now, he was fairly convinced that Bruce had some connection to Batman, but to Harvey, the notion of one man being able to do everything the Caped Crusader did was almost impossible.
Harvey's dream of a world where everything went his way explained his rationale: that there isn't just one Batman, but multiple. In his fantasy, the Dark Knight is actually multiple highly trained individuals funded and supplied by Wayne, but also authorized by several of Gotham's elite from the mayor to wealthy businessman and even James Gordon. This part of the dream explains why Batman always seems to get away with vigilante activity; because it's not really vigilantism, just black ops performed by the city's ruling class.
In this world, Bruce only confessed to this because Harvey had accomplished everything that all of Wayne's accomplices could not. Although he was a little off with the exact details, Harvey, through less than a handful of fact-finding missions nearly solved the puzzle of who Batman is without any direct contact with the Dark Knight.
The idea Dent had isn't unprecedented. When Bruce Wayne realized that he could not not protect the entire world alone, he founded his own organization to keep the peace: Batman Incorporated. It was quite literally everything Harvey's dream laid it out to be, just on a global scale.
Batman would fund and coordinate multiple highly trained people into a network of Batmen to keep the world safe. Bruce Wayne even publicly admitted that he was funding Batman for years, a clever way of hiding his identity while also placating doubters of his innocence, although Harvey seems to believe that such a group only makes things worse.