The "strongest" argument they have is "why not Biden" and that one is actually paper thin for two reasons.

First of all, Biden cooperated and turned over the documents he found.

Secondly, if it does turn out that Biden took documents and hid them then he should certainly be prosecuted for this.

So even at most "why not Biden" isn't a defense for Trump.

Maybe he should try the "affluence defense". He has spent his entire life getting everything he wanted and getting out of any trouble he finds himself. It's no wonder he thought he could cheat his way into the White House and steal State secrets.

The key issue is intent. They have Trump on tape saying that he is currently in possession of classified material that he's not supposed to have and admitting that it has not been declassified. They also have text messages from his employees relaying his orders to hide boxes of classified documents right before someone came to collect them to be returned. It's rock-solid intent.

The only way to negate that would be to show that his mental health has deteriorated to the point that he didn't appreciate his words and actions. He's saying and doing things with no real intent behind it, because he's completely lost his mind. There's no way Trump co-signs that defense.

Maybe they can get him declared incompetent to stand trial, that's the best defense I can think of.