Nolan's Dark Knight: Why would Dent's convictions be overturned?

Question

In The Dark Knight, when Batman interrupts (what would probably have ended in) Dent killing Thomas Schiff, Batman tells Dent that if people saw him doing this, then all of the criminals he'd locked up would be released. Now, I'm not a lawyer or even a student of the law, but it seems to me that the post-facto actions of counsel would have no bearing on a verdict rendered.

Is there some legal precedent of which I'm ignorant that the filmmakers might have been drawing from?

BATMAN : You're the symbol of hope that I could never be. Your stand against organized crime is the first legitimate ray of light in Gotham for decades. If anyone saw this, everything would be undone-all the criminals you got off the streets would be released. And Jim Gordon will have died for nothing.

Answer 1

From a legal perspective, it's worth noting that the criminals the Mayor is referring to aren't those that Dent has already convicted in the past but the 549 criminals that Dent convinced Commissioner Gordon to arrest in scene 91.

Not only are the criminals involved not yet convicted (far from it) but Dent makes it abundantly clear to the Mayor that the only reason why Judge Surrillo is entertaining this "farce" is because of Dent's personal involvement:

MAYOR : 549 criminals at once?! How did you convince Surrillo to hear this farce?

DENT : She shares my enthusiasm for justice. After all, she is a judge.

MAYOR : Even if you blow enough smoke to get convictions out of Surrillo, you'll set a new record at appeals for quickest kick in the ass.

DENT : It won't matter.

The whole thing is a dramatically risky move (basically a gigantic nuisance suit) and one that seems doomed to failure. Since all parties recognise that the majority of the criminals will be out on the streets within a few months anyway due to lack of evidence, the Mayor states that he is relying on Dent's public popularity to smooth over the obvious recriminations about the wasted time, effort and cost.

No Dent (or if Dent turns out to be dirty) = no trials.

MAYOR : The public likes you, Dent. That's the only reason this might fly. But that means it's on you. They're all coming after you, now. Not just the mob... politicians, journalists, cops - anyone whose wallet's about to get lighter. Are you up to it?

(Dent smiles)

You better be. They get anything on you... those criminals will be back on the streets...

Answer 2

Aside from referring to Dents plan to clean up the streets of those 500+ criminals as mentioned in the other answer, from a legal perspective, post conviction relief can be found in many forms. Prosecutor misconduct would be one of them.

While my limited searching didn't pull up anything, proof that a prosecutor (or witness/police, judge, etc) was racist, took bribes, sexually assaulted/black mailed, intentionally hid evidence, tampered with evidence, etc, would cause every single smart person affected by that prosecutor to seek post conviction relief appeals.

This would normally require A tangent to the person's role as prosecutor.The severity of the crime is also a factor. a bar fight wouldn't be enough. In real life, any time a cop is shown to (excessively) lie about a case, has been used in these appeals. A recent issue in NYC was a drug testing lab tech who faked samples and said they tested for coke, which the state used as evidence. Once the evidence was shown to be fake, every drug case where that evidence was used was appealed.

Dents extra judicial "execution" would be enough to grant hearings of those post conviction relief appeals. Dents credibility would be trashed. As to how many would be successful would be anyone else's guess.