Step 1 in dealing with election fraud deniers
Since many of them try to stop any discussion with "It's all been litigated and judges threw out all the cases".
Judges did not throw out all the cases, and in fact Trump’s legal team prevailed in many cases. The problem is that none of the cases that were allowed to proceed anywhere dealt directly with the fraud, or with a forest level look at anything. One of the lists of cases can be found here:
https://www.scribd.com/document/491936697/2020-Election-Court-Cases#from_embed
Rather than try to point out how none of these cases dealt with the big picture, and/or getting into the myriad of details making it obvious fraud was involved, just keep it simple. Ask the fraud denier if they know of any previous court case that they have watched on TV, heard on the news, or read about, whether civil or criminal, where eyewitness testimony was not allowed, and the evidence was also not directly reviewed and discussed.
Unless they are horribly ignorant and have never seen a trial or any part of a trial, or they are simply afraid to even try and defend their beliefs (And this reality is all too common with left wingers on almost anything they believe.), they will admit that they have seen witnesses testifying in all trials that are newsworthy. So ask them to explain why no court anywhere in the U.S. heard testimony from any of the thousands of witnesses who filed sworn affidavits that they saw various types of cheating going on during the 2020 election.
You can use any highly publicized trial that they have seen to make the point. If you’re old enough, imagine O.J. Simpson’s trial with no one testifying. No experts, nobody who witnessed anything, the case was just decided by the judge based on legal filings and some brief oral arguments by the attorneys on either side. Ditto for the very recent Kyle Rittenhouse trial. Think about all those witnesses, including Rittenhouse himself, being prohibited from testifying and the case being sent to the jury just based on words on paper typed up by attorneys on either side.
The truth is that no 2020 election related case that directly addressed illegal ballots and/or that addressed the voting machines and backend servers in 2020 was decided based on evidence. In those cases where Trump lost, lack of standing, lack of jurisdiction, and similar legalese was invoked nearly every time to simply dismiss the case before any discovery, any testimony, or any expert review of evidence could ever take place.
It’s a longshot to get a left winger to admit that they are wrong, but simplifying it down to the fact that no eyewitnesses were ever allowed to testify may at least get them thinking.