Not OT At All

Not entirely OT, but I've got a question maybe you can answer, acknowledging your crim law experience is limited (if that's the right word?) and the answer(s) might inherently be too much in the weeds. 

What rules govern what a judge can/must allow as evidence for a defense? 

I've seen a lot of Trumpers crow about "now he can subpoena people and prove 2020 fraud" so he was "justified" (not a legal term AFAIK) in his little schemes. My trial experience is what I've seen on TV, but it doesn't sound proper as a layman, although I don't even know where to start with all the criminal procedure rules and whatnot. No answer required, of course. Just throwing it out there (maybe some interesting blog fodder?). :-)

Basically: no.

The governing idea is evidence relevant to a defense to the charges made by the prosecution is admissible.

Not only does it not matter whether Trump truly believed he was cheated, it doesn’t matter if he can prove it. The best analogy is to a bank robber who thinks the bank fraudulently took his money. That belief may be sincerely held, but it’s no defense to a robbery charge. (Or my one criminal client who sincerely believed he wasn’t in possession of a firearm because he didn’t own it. We didn’t even try to argue that point in court. We knew it was no defense.). Since it’s not a defense, the short answer is: Trump won’t be allowed to put in any evidence about it.  Because it’s irrelevant.

My expectation is Trump’s lawyers never try to raise that defense, and the prosecution keeps it out in a motion in limine (a pre-trial order on issues that can’t be raised before the jury).

There are Federal Rules of Evidence. The rule on hearsay is practically a law school course on its own. That’s complicated beyond explaining easily (the exceptions to the rule are legion, because they are so many). The basic purpose of the rules is to allow verifiable information (think a research paper or something in a professional journal), and disallow that which can’t be verified; like hearsay. The Rules also exclude irrelevant evidence, so the jury can decide the facts from as sound a set of data as possible.

Trump tried 60+ times to reverse the election results in court. He can’t relitigate those failures. The issue of stare decisis doesn’t directly apply, but the judge is not going to try the criminal case and all of those cases, or even one of them. That matter is utterly irrelevant. So all the evidence Trump would have to bring in to prove fraud (remember, he can’t just say it over and over again) couldn’t come in. Not to mention it doesn’t exist.

Remember those 60+cases? Remember not a one of them got past a motion to dismiss? In no case was evidence presented, because there was no evidence to present. The case Giuliani personally argued was, by his own admission in open court, a fishing expedition. Those suits were filed purely to find evidence of fraud, because they couldn’t find any. Trump probably imagines he can just scream “FRAUD!” and he wins. But that’s not evidence (it’s not even hearsay), and he can’t testify without being cross-examined. And if he’s cross-examined, he’s toast.

And even if he does testify, all that “evidence” he thinks he has is inadmissible.

It’s simply not going to happen. Because that’s not how criminal trials are conducted. What Trump’s imagining is a countersuit, because he has plenty of experience in civil court, but none in criminal court. Even if he was in civil court, he couldn’t do it as a countersuit. Who would he sue, that he hasn’t sued already? Stare decisis is a real problem.