1) Trump wants to raise his election-fraud allegations as impeachment defense. But even if he could prove that fraud (he can't), it would only be relevant as an affirmative defense of necessity. If he committed no crime, then the election-fraud is utterly irrelevant to any ...

2) issue in impeachment. But if he did commit crime (e.g., sedition conspiracy, inciting riot), election-fraud MIGHT be relevant if it excused or justified the crimes he committed. Defense of necessity can be raised if you must commit lesser crime to avoid commission of greater

3) crime, e.g., stealing $ from bank where you work because bad guys kidnapped your kids & threaten to kill them.
But in raising this defense, you are admitting, as prima facie matter, that you committed the crimes you claim are justified/excused. It is then your burden to

4) prove your criminal conduct was justified/excused. Violent overthrow of govt is NOT lesser evil of election-fraud; it's NOT necessary even if you believe election-fraud occurred. Your remedy for fraud is to file suit, and prove, in court (which Trump was woefully unable to do)

5) This may provide a good reason to delay the trial for a month or more, which would give Ds a chance to prep an even stronger case & give the public more chance to learn & see what happened on 1-6. I suspect pleas will start coming soon.