He lost the rest of the week's news cycle as well, in the most petty and self-destructive fashion.
Trump suffered a meltdown as spectacular as the one after the Democratic National Convention when he picked a fight with the father of a dead soldier.
That earlier malfunction contributed to an August-long poll bounce for Clinton and both events have dragged Trump's psychological frailty blinking into the sunlight.
Looking back, the first presidential debate unfolded like a sword fight. Trump came with a charge, raining blows on Clinton, but was worn down to a flailing mess by her mix of skill, steel and smirk.
Having slashed effectively at Trump's self-image with a reference to the support he received at the start of his business career, Clinton drew blood with various attacks on tax, housing and birtherism before finishing her opponent off with a carefully planned trap.
She mentioned previous derogatory comments Trump had made about women and raised the case of Hispanic former Miss Universe Alicia Machado as someone the Republican had insulted as overweight, calling her "Miss Piggy" and "Miss Housekeeping". The Clinton campaign quickly released a video of Machado talking about her treatment. The issue both reminded women of why they overwhelmingly prefer Clinton to Trump and served as a call out to Latino voters as well.
Trump should have just left it there but instead he talked about it for days, escalated it with claims of a Machado sex tape and wrapped it all in with surrogates' shots at Clinton for her husband's previous infidelity. Trump even fired off middle-of-the-night local time tweets fuelling the feud.
Former Obama campaign strategist David Axelrod told CNN: "[Trump's] very reactive. The Clinton folks figured that out, they were pushing his buttons all throughout that debate and he is still reeling from that".
Axelrod explained: "After that debate, my question was how will he react to this general verdict that he had lost the debate, because ... Trump pictures himself as a winner. That's his whole image. I'm a winner. Everybody else are losers. And whenever he's depicted as a loser, it just drives him off the deep end."
This has wider implications than just Trump's personal feelings.
Atlantic writer Conor Friedersdorf tweeted: "Hillary managed to get inside Donald Trump's head for 3 days. He did everything she would've wanted. How many days could Putin play him?"
Vox's Ezra Klein outlined that point further.
Trump's depiction as unpredictable is a misreading, he said.
"His behaviour, though unusual, is quite predictable - a fact the Clinton campaign proved by predicting it. His actions, though beyond the control of his allies, can be controlled by his enemies - a fact the Clinton campaign proved by controlling them. So far, this has played out, within the safe space of a presidential campaign, as farce. If Trump were to win the White House, it would play out as tragedy."
Klein added: "We're now six days beyond the debate. And Trump is still finding new ways to spring and re-spring Clinton's trap on himself.
"Imagine it wasn't Hillary Clinton trying to bait Trump into attacking Alicia Machado, but Isis trying to bait Trump into attacking Iraq, or Vladimir Putin trying to bait Trump into breaking with Nato, or Angela Merkel trying to bait Trump into isolating the United States before a key vote at the United Nations, or China trying to bait Trump into giving them an excuse to assert their claim over Taiwan."
The debate and its fallout have already clearly given the Clinton team fresh momentum.
Long term electoral patterns have been in Clinton's favour, but a more scripted Trump in September came close to Clinton's national poll average lead and eroded the Democratic poll position in several swing states.
Now the evidence is Clinton has received a slight national poll bounce - 538's Nate Silver said yesterday that he believes her lead to be about 4 per centage points - while a batch of about a dozen battleground state polls have reinforced the castle walls.
The question is what happens now? Can Clinton keep it going? Can Trump fight back? Can Trump stop alienating people he needs to vote for him? Does he even want to?
The hypocrisy of a bunch of known previous adulterers - Trump and surrogates Newt Gingrich and Rudy Giuliani - dredging up Bill Clinton's baggage to attack his wife is kind of mind-boggling, but only if you expect Trump to actually want to expand his demographics beyond largely white, male support.
Trump essentially appears to want to talk to the converted and for others to fall in line.
Clinton has her own problems. She is struggling to turn Millennials - the 18 to 35 age group that makes up 25 per cent of the electorate - into dependable supporters with many considering third-party candidates.
The vice-presidential debate midweek offers the Republican campaign a chance to present a calmer, more mature face in the form of Mike Pence.
However next week's second presidential debate could be a difficult assignment for Trump.
It will be in the town hall format, meaning a mixture of moderator and audience questions. Candidates will have to be highly disciplined about their demeanour and interactions with the audience.
That will likely make it difficult for Trump to launch a sustained attack and will allow Clinton to display her greater policy knowledge.
A more relaxed Trump could use humour to his advantage but there's always the likelihood of the Republican being jabbed into a damaging loose line.
And then there's always the chance of another mad meltdown before then.
Josh Marshall of Talking Points Memo tweeted: "Question is whether there's time left for Trump to hire new campaign staff, get teleprompter discipline and meltdown again b4 election day."
