It occurs to me that our current president has engaged in a rather large number of actions that are unprecedented by any other president, as well as some only rarely happening and not for a long time, many of which for nearly all other politicians would make them viewed as simply unacceptable for being a president. However, with him, there have been so many that people simply ignore most of them as being, "Oh well, there goes Trump again being himself."
First of all I do not have a full list of his norm-shattering precedents. In googling I saw an article that proclaimed he had violated 20 "presidential norms." But it was behind a firewall. And even though these may have been norms, it may be that other presidents violated at least some of them. Anyway, here goes a a partial list.
The first president to be accused of violating the emoluments clause of the Constitution. Actually there are two such clauses, the more famous one involving foreigners buying off a president, and then a domestic one as well. He has been accused of violating both of them on numerous occasions, although never formally convicted of any, with a couple of law cases in court, mostly involving just the Trump Hotel in Washington. But the list of likely violations is long and involves many properties and forms, with at least 22 foreign nations reputedly involved. The funny thing is that just because something is forbidden in the Constitution does not immediately make it specifically against the law. In any case, it remains that somehow no other president has even been accused of violating either of these clauses, but Trump is pretty clearly massively guilty of actually violating both of them blatantly and numerous times, with only the mildest outcry even from his critics.
He is the first president to have been legally charged with engaging in racial discrimination. This would have been in 1973 right after he began working for his father's real estate business. He was not found officially guilty but had an out of court settlement in which he was ordered to stop doing what he had been doing. As it was, the Justice Dept later accused him of doing exactly that, returning to his previous racially discriminatory practices, after which he seems to have more or less stopped. Of course there are past presidents with much worse race relations records than his, such as outright owning slaves as did most of the Founding Fathers from the South. But in those days what they did was legal, if immoral.
The first president to be sued for breach of contract. I do not have the exact number of times that he has been so sued, but it is a lot. As it is he has been involved in over 3500 lawsuits, although more often than not with him as the plaintiff. This is more than the combined lawsuits involving the top five construction businesses in New York City. Many presidents have been sued for a variety of things, such as defamation or policy actions they engaged in as president. Not a president, but Hillary Clinton has been sued about 900 times, mostly by people upset with policy actions by her, never for breach of contract, despite all her and Bill's Whitewater shenanigans. Somehow I would think that repeatedly violating something so basic as simply keeping one's word in a business contract would make one ineligible to be president, but I think many of his followers inaccurately think this is a sign of him being a "good businessman," a reason many people voted for him in 2016 reportedly.
The only president to go bankrupt more than once, in his case six times, another matter suggesting he has not been as good a businessman as many think he has been. Four other presidents have gone bankrupt, each of them once and in all cases due to cosigning for loans where the person they cosigned with engaged in conduct that led to the financial problems that dragged the president into having to declare bankruptcy. Those four included some distinguished ones: Thomas Jefferson, Abraham Lincoln, U.S. Grant, and William McKinley. Of course Jefferson was a profligate spender and poor money manager, dying deep in debt. Lincoln's wife, Mary, was a notorious overspender. Grant generally had poor experience in his business affairs, with his bankruptcy occurring after he left office, with some attributing his drinking to his poor business judgment.
The only president to suggest that those who died in military service for the US were "losers" and "suckers," apparently the last straw that led John McCain's widow, Cindy, to support Biden for president.
The only president to publicly lie in office more than 1000 times. The latest count for him now exceeds 23,000 times, and this is in a bit less than one term. Actually I could not find an exact count for any other president basically because they did it so infrequently nobody seems to have kept count, so it is possible I am wrong on this. But certainly nobody came remotely close to him in his lying rate. Digging around it seems that his closest rivals were actually a predecessor-successor pair: LBJ and Nixon. LBJ notoriously repeatedly lied about the Vietnam War, a very serious matter, with the Gulf of Tonkin incident at the top of the list. Of course, Nixon repeatedly lied about Watergate. Almost certainly every president lied while in office, with the lies ranging from serious to strategic (FDR in WW II) to political to sexual (Bill Clinton) to careless unawareness of facts (Reagan) to just frivolous. In digging into this I found an article excoriating Obama for supposedly lying 37 times about details of the ACA, with the article, written by a critic while he was in office, suggesting that this would ruin the entire rest of his presidency and nearly rose to the level of impeachability. Oh how we have moved on.
Which brings us to refusing to concede defeat when an election was called against him by leading observers. We need to be clear here that a number of elections were not called for a long time and involved difficult and even extra-constitutional machinations, including 1800, 1824, 1876, and 2000. But in all those cases once the unusual delays ended in determining the winner, the loser conceded forthwith without further efforts to undo the outcome. As of almost a week ago Trump did finally allow the GSA to allow transition activities to begin, but only because he could still contest the election outcome, which he continues to do, with I gather a majority of his backers still supporting his efforts, even as nearly all his lawsuits have failed and enough states have formally certified their results to guarantee that indeed Biden will have more than 270 electoral college votes.
This leads us to a last item, not completely unprecedented and not yet done by him, but not done since 1869, and on twice before then, in 1801 and 1829 some in connection with those dragged out contested elections, but with this one a pretty clearly solid win for Biden. It would be if he refuses to attend and participate in the inaugural of his successor. Even Herbert Hoover did so in 1933 for FDR in what many have identified as the last really hostile and difficult transition, although Hoover did not contest the landslide outcome against him. Trump may yet change his mind, but current reports have him considering not only not attending but holding a rival event at the same time, a rally in which he would declare his candidacy for 2024, and if he does that, it would be another unprecedented action. John Adams, John Quincy Adams, and the also impeached Andrew Johnson did not do anything like that when they did not attend the inauguration of their respective successors. And if somehow Trump does not attend, expect his fervent followers to be all impressed rather than duly shocked.