You have to hand it to the Republicans. They have a fiendish ability to rehabilitate the toxic reputations of their previous presidents by having the party’s new nominee be even worse.

Let’s start with Nixon. Often unhelpfully moist, he was ruthless, conniving, unstable, dissembling, and forgiving of criminal acts as long as they were perpetrated on his behalf. Promised to end the Vietnam War, which he did by extending it five more years, bombing other countries, and adding the names of more than 21,000 American soldiers to the death toll. Anti-Semitic and racist. Hostile to the press and criminal to the opposition—Watergate was a catalogue of assaults on decency, the law, and the Constitution. Only president in history to resign, and when he did, the sane and the decent between and beyond our borders heaved a sigh of relief: “He’s gone, thank God, the worst is behind us.”

Seven years later, the G.O.P. gives us Reagan. The intellectual curiosity of a shingle. The work ethic of a no-show Mafia union hire. A coy race-baiter who initially opposed a holiday for Martin Luther King Jr. and once mistook a Black member of his Cabinet for a Black mayor. Ignored AIDS except when his wife intervened on behalf of their friend Roy Cohn, who was so evil Satan has a poster of him over his bed. Tells Holocaust survivor and Nazi hunter Simon Wiesenthal that he personally shot footage of the liberation of the Nazi death camps even though he spent the war in Culver City, California. Trades arms for hostages after saying he would never negotiate with terrorists. Can’t go two days without telling a lie. And suddenly you remember Nixon and think, “He created the E.P.A. He signed arms-control treaties and opened relations with China. He did not think books were just a type of set decoration. He served with distinction in the navy, and he was unquestionably intelligent, however perversely he misapplied that intelligence. And his daughters adored him—that has to count for something. Reagan’s worse. We’ll never do worse than Reagan.”

They have a fiendish ability to rehabilitate the toxic reputations of their previous presidents by having the party’s new nominee be even worse.

Until George W. Bush. The closing argument against legacies. Cocky, smug, inexplicably self-admiring, he inherits a booming economy with a budget surplus that he turns into the Great Recession and a bottomless deficit. Ignores the warnings before 9/11, lets bin Laden escape, and attacks the wrong country. Does not appear to believe in separation of church and state. Denies the climate crisis and vetoes stem-cell research. Is largely incapable of speaking in sentences. Except for the admirable work he did for AIDS in Africa, by the time he leaves office almost everything he touched is worse for his having touched it.

And suddenly Reagan looks better. “Yes, he was an idiot, and narrow in his interests, but he could give an inspiring speech when needed, and he knew how to make deals, whether with Congress or Gorbachev. Best of all, despite his tough talk, he largely resisted sending American soldiers to war. He would never have done what Bush did in Afghanistan and Iraq. He was terrible in many ways, but he’s better than this preening, over-his-head, out-of-his-depth frat boy. Yes, Bush is the worst—we’ll never do worse than George W. Bush.”

And here we are with the steatopygic Donald J. Trump, who sinks to every occasion. In a negative version of a miracle, he has all the worst qualities of his predecessors and none of their merits. He is inconceivably more contemptuous of the law than Nixon was but lacks his intelligence and strategic thinking. He is lazier and less mentally engaged than Reagan but without Reagan’s affability. He has all of Bush’s entitled self-approval with none of his underlying good nature. He inherited a strong economy that he promised to run like he ran his businesses. Which he did—we’re hemorrhaging money.

Temperamentally Trump is the offspring of the vicious and mendacious Nixon, but we owe the calamity that is the Trump administration to Ronald Reagan. It was Reagan’s open contempt for government itself that made Trump possible—who better to head up something that you find beneath contempt than someone who is himself beneath contempt? What better choice to run the hated federal government than someone who will run it into the ground? Reagan took the old G.O.P. idea—that smaller government is better—and twisted it into the poisonous proposition that government of any size is bad. He exhorted people to think of government as not the solution to problems but as the problem itself. Selling himself as an idealist, he disabused people of their ideals. (How ironic that Reagan found his success by demonizing the Soviet Union, and now his shining city on a hill is run by a Russian puppet.)

He inherited a strong economy that he promised to run like he ran his businesses. Which he did—we’re hemorrhaging money.

Though I am a Democrat, I grew up in a Republican family. Here’s what being a Republican meant to my parents: Small government. Low taxes. International alliances, but let’s not get all kissy with the U.N. They believed in God and went to church, but if they had a cough, a rash, or a tumor, they went to the doctor. They didn’t think of science and faith as incompatible—they believed God created scientists too, and doctors know science better than ministers do. In polite conversation they were against abortion, but in reality, if an unmarried teenager got pregnant, abortion was on the list of things to be discussed. They were excited about the moon stuff but generally weren’t for massive government programs unless it benefited them in some way. (My father was in the oil business and supported any and every advantage the oil industry could get.)

Most of all, though, they believed that the G.O.P. was the more patriotic party. (We heatedly disagreed about this and found a new level of happiness when we agreed to stop talking about it.) They thought that the Democrats were soft on Communism and weak on law and order. (Unsurprisingly, the two presidents most identified with the phrase “law and order” are the most corrupt: Nixon and Trump.) My parents loved America, and so they were Republicans because they felt Republicans loved America the most.

Mom and Dad died several years ago, and not a day passes when I don’t miss them. But I am profoundly grateful they were spared having to see this squalid thing the G.O.P. has become: a crime-blind, thug-hugging, NATO-baiting, enemy-embracing, transactional fountain of hate.

The rest of us aren’t so lucky.

Douglas McGrath is a filmmaker and playwright. He lives in New York City