“Res Judicata”—Latin for “the thing has already been decided (adjudicated)”—is a principle of litigation holding that, when an issue between litigants has already been decided in one lawsuit, that determination stands for any subsequent lawsuits involving the same issue and parties. It cannot be relitigated.
The classic, true story, case illustrating this principle is: Dude burns his house down and is convicted of arson. He subsequently sues his insurance company. Res judicata: It’s already been determined that he was the perp, which invalidates his claim on the insurance policy.
In their answer to the Article of Impeachment, Trump’s team asserts that any reasonable person would have doubts about Biden’s election. Sorry, the courts have already answered that question.
Trump’s stolen election claim was addressed by dozens of courts, both state and federal. The evidence (and lack of evidence) was reviewed and found to be so insufficient, tenuous, debunked and/or lacking credibility that judge after judge determined, as a matter of law, that the claim is bunk. “As a matter of law” means, the evidence is so clear that the judge could rule on the merits, no need for a trial.
You had two and a half months and many dozen opportunities to present your case, Ex-President Donald. You lost. You cannot relitigate this claim in your impeachment trial. You cannot raise it in your defense.
Now to be sure, I do not know if there are special rules or precedents that would make this principle, res judicata, inapplicable in a Senate impeachment trial. But it is a principle founded on universally accepted standards of fairness; I don’t see why it wouldn’t apply.
There's no reason for the Senate and the country to waste more time and energy on the Big Lie. It’s been decided.