It has been nearly four years since Donald Trump broke all the rules of presidential politics with his campaign announcement at Trump Tower. Speaking to a crowd that included paid actors, he began on June 16, 2015, by saying with unintended irony, “There’s been no crowd like this,” then went on to extol his air conditioning (other candidates didn’t have it, and “they sweated like dogs”), his golf courses (“the best … in the world”), his lobbyists (“can produce anything for me”) and his net worth (“well over $10 billion”).

President Trump and French President Emmanuel Macron arrived at a commemoration ceremony for the 75th anniversary of D-Day in Normandy on June 6. (Reuters)

In between these advertisements for himself, Trump berated two of America’s biggest trade partners — China (“They kill us”) and Mexico (“When Mexico sends its people, they’re not sending their best … They’re bringing drugs. They’re bringing crime. They’re rapists”). To make his case that “the American dream is dead,” he relied on fraudulent statistics (e.g., “our real unemployment is anywhere from 18 to 20 percent”). He then made a bunch of promises that everyone knew he could never carry out — “I will build a great, great wall on our southern border, and I will have Mexico pay for that wall,” “I’ll bring back our jobs from China, from Mexico, from Japan, from so many places.”

Sixteen months after this jaw-dropping performance, he was elected to the same office as Washington, Lincoln and Roosevelt.

At one time, it was naively assumed that, in the unlikely event that he won the presidency, Trump would tone down the lying, the bragging, the nativism and the insults, and act more, well, presidential. Trump himself promised that “with the exception of the late, great Abraham Lincoln, I can be more presidential than any president that’s ever held this office.” Another lie. We are more than halfway through Trump’s term, and his behavior remains as “unpresidented” as ever.

Just look at how Trump behaved in his trip to Europe to commemorate D-Day — which he seems to think stood for “Donald’s day.” Trump managed to shatter one norm of presidential decorum after another.

Don’t engage in domestic politics overseas — and don’t engage in name-calling. During a Fox News interview at the American war cemetery in Normandy — can there be a bigger sacrilege? — Trump called special counsel Robert S. Mueller III a “fool” and House Speaker Nancy Pelosi (D-Calif.) a “disaster” and “Nervous Nancy.” Earlier on the trip, he called Senate Minority Leader Charles E. Schumer (D.-N.Y.) a “creep” and singer Bette Midler “a washed-up psycho.”

Don’t lie. Trump made one false claim after another. The United States doesn’t have the “cleanest air in the world,” and it hasn’t “gotten better” since he became president. Trump doesn’t have the highest level of support ever among Republicans. Climate change doesn’t go “both ways.” Trump has attacked John McCain, and he did call Meghan, Duchess of Sussex, “nasty” — it’s on tape. Oh, and it wasn’t “fake news” that massive crowds demonstrated against Trump in London. The actual “fake news” was Trump’s claim that “thousands of people” were cheering him in London.

Don’t intrude into other democracies’ politics. Trump all but endorsed Boris Johnson’s quest to become prime minister of the United Kingdom, suggested that populist rabble-rouser Nigel Farage should be appointed to negotiate with the European Union, and said that Britain should pursue a “hard Brexit.” He even proposed including the National Health Service in trade talks. In his meeting with Irish Prime Minister Leo Varadkar, a Brexit opponent, Trump recommended a “wall” along the border between the Republic of Ireland and Northern Ireland — the last thing that either side wants. In his defense, Trump was probably not trying to micromanage Brexit negotiations; he simply didn’t know what he was talking about, as usual.

Don’t mix government business with personal business. Trump made two separate trips, at considerable taxpayer expense, to spend the night at Doonbeg, his money-losing Irish golf resort. He even tried to get Varadkar to meet him there — a request the Irish prime minister refused. Trump’s visit provided a publicity windfall for the golf course and de facto put the weight of the American presidency behind the Trump Organization’s requests, still pending with the local authorities, to build a sea wall, more than 50 guest cottages and a ballroom. Trump’s sons, Eric Trump and Donald Trump Jr., who run the Trump Organization, even used the occasion to go on a pub crawl to win over the locals.

Despite all of these faux pas, offenses, inanities and improprieties, the trip was, by Trump’s debased terms, a relative success. (Meaning: He didn’t throw food at the queen or moon the prince of Wales.) He said so himself. Or rather, he quoted Sean Hannity saying so: “The President has received glowing reviews from the British Media.” Of course, that was another lie: the British media coverage was scathing and contemptuous.

Trump’s buffoonery is bad enough at home; it’s especially embarrassing when he is supposed to be representing the entire country overseas. He makes me ashamed to be an American. But don’t say you weren’t warned: From the start of his campaign in 2015 until today, Trump has been nothing if not consistent in his contempt for behavioral norms. It’s not his fault that he is so awful; after 72 years, he can’t help himself. It’s our fault that we elected him and might reelect him in 2020.