Republican presidential nominee Donald Trump. Credit:AP Could he be persuaded to practice? Would he reassure undecided voters that he could rise above the fray when the situation demanded? What tactics might he use to throw a seasoned performer like Clinton off balance? From his first rambling answer, however, it became clear Trump had no tactics. His presence on the stage was as a whirling and mercurial storm of ego and bluster. His answers often incoherent, he delivered few sustained thoughts on any subject matter and responded to requests for policy detail from moderator Lester Holt with lists of problems facing America often unrelated to the question at hand. Did he have a plan for jobs? America's factories are moving to Mexico. How can America improve race relations? Inner cities are dangerous. What to do about the growing threat of cyber-warfare? Trump's 10-year-old son Barron is a computer whiz. Yet far more worrying than Trump's lack of policy detail was his incurious treatment of subjects about which no leader can afford to be glib.

Illustration: John Shakespeare "I haven't given lots of thought to NATO," he said when questioned about statements suggesting he would refuse to honour the 67-year-old military alliance that plays a key role in US foreign policy. On the Middle East, he reiterated his belief that the US should have taken Iraq's oilfields after invading the country, an act that would constitute a war crime under the Geneva Conventions. He talked up a New York City policy permitting police to randomly search members of the public, then argued with Holt about whether or not the proposal had already been declared unconstitutional. Trump was wrong – a judge found New York's stop-and-frisk tactics to be racially discriminatory and ended them in 2013. But even if American voters could tolerate Trump's ignorance and policy vacuity, he demonstrated better than ever that he is fundamentally incapable of behaving in a manner befitting the nation's highest office.

From his entry into the presidential race, Trump has found success by bullying his opponents. Whether in the primary debates, in interviews with journalists, or on stage at the Republican National Convention, his strategy has been to exert dominance over anyone who challenges him, winning by declaring himself the biggest, toughest guy in the room. It worked when he had to stand out amid a gaggle of primary candidates vying for attention or at his own stage-managed events. But one-on-one against an opponent on equal footing, his thuggishness was revealed as childish. When running for the New York Senate seat in 2000, Hillary Clinton turned the tables on her opponent Rick Lazio; his attempts to dominate her, including standing over her and insisting she sign a campaign finance pledge, struck voters as menacing. Lazio lost the election by 12 points. Trump should have been aware of this danger. But from the outset of the debate, he failed to even be civil to his opponent, addressing her as "Secretary Clinton", then undercutting the courtesy with a patronising aside: "Is that OK? Good. I want you to be happy." It got worse. For most of the debate, he didn't even address Clinton by name, referring to her again and again as "she" and interjecting "wrong" over and over again as she was talking. The website Vox calculated he interrupted her 51 times over the course of the 90-minute debate.

Trump couldn't be polite to Holt, either, squabbling with the moderator on the too-rare occasion he was fact-checked or asked to stay within his speaking time. Controlling Trump wasn't an easy task for Holt, but he permitted the Republican notably more leeway than he did Clinton. Trump's petulance was compulsive; it seemed as if he was constitutionally incapable of permitting any attack to go unanswered, to let anyone else have the last word. Polls over coming weeks will show whether this extraordinary display has weakened his standing with the voters, but it should. Donald Trump proved today beyond a doubt that he is a dangerous, ignorant, and profoundly unpleasant man. Americans now have all the evidence they need to declare he should never be president. May they return him this November to his tacky hotels and reality show melodramas and put him out of our lives for good. Jonathan Bradley is a writer on US politics and culture. Twitter: @_jbradley