The bizarre gaffe that sealed Biden's fate... his dirty tactics exposed... and his botched response to the most important question of the night: Your essential guide to all the debate drama, by SCOTT JENNINGS

President Joe Biden's senior advisers, aides and the First Lady have a lot of explaining to do.

For weeks, they've assured America that the Democratic party's 81-year-old presumptive nominee was fighting fit and capable, not only of serving out his term but carrying on four more years.

Americans were told not to believe their lying eyes, that the man they watched freeze at public events and ramble incoherently when speaking extemporaneously was actually sharp as a tack behind the scenes.

Well, where was that man at Thursday night's debate?

The future of Joe Biden's campaign rested on this single test – the first faceoff of the 2024 election – and he patently failed.

Ironically, this was the showdown that the Biden campaign wanted.

The future of Joe Biden's campaign rested on this single test – the first faceoff of the 2024 election – and he patently failed.

President Joe Biden's senior advisers, aides and the First Lady have a lot of explaining to do.

In May, his team released a video – spliced together from half-a-dozen takes – of the President challenging Trump to debate.

'Make my day, pal,' Biden swaggered.

Well, Mr. President, you've gotten what you asked for.

These are the key moments that now have Democrats in full-blown panic – and wondering if there's time to swap out their candidate for another before voters go to polls in November.

MOUTH AGAPE AND CONFUSED

From the minute President Biden shuffled onto the stage in Atlanta, millions of Americans watching knew something was wrong.

His appearance was, frankly, shocking – mouth agape and eyes hooded.

Then, he opened his mouth – and it was worse.

His voice was weak. His train of thought wandering.

Throughout the 90-minute debate, he was often difficult to hear and completely unintelligible.

From the minute President Biden shuffled onto the stage in Atlanta, I and millions of people watching at home knew something was wrong.

At one point, Biden inexplicably careened from discussing abortion policy to illegal immigrant crime.

'There's a lot of young women who are being raped by their... by their in-laws, by their... by their spouses, brothers and sisters, by... just... it's just... it's just ridiculous,' he said.

Ridiculous, indeed.

Even Democrats were astounded.

'There is a sense of shock about how he came out at the beginning of this debate about how his voice sounded,' said former Obama campaign chief strategist David Axelrod. 'He seemed a little bit disoriented.'

JOE GETS NASTY

As the debate wound down, President Biden got wound up – flashing anger and insults.

He predictably slammed Trump as a 'convicted felony,' but then became disturbingly more specific.

He first falsely claimed that Trump owes 'billions of dollars' in civil penalties for 'molesting a woman in public.'

In fact, Trump is appealing a defamation lawsuit he lost against writer E. Jean Carroll, who alleged that he sexually assaulted her in a department store decades ago.

As the debate wound down, President Biden got wound up – flashing anger and insults.

Then Biden went further and spat at Trump that he had 'sex with a porn star,' adding 'while your wife was pregnant.'

These allegations against Trump are well-known. They won't do much to change any voters' minds. 

Though, they do make Biden appear pathetically desperate.

DON DELIVERED

From the economy to the border, to foreign policy and health care, Trump delivered the most effective and impactful lines of the night.

The former president effectively contrasted his stewardship of America's longest war with Biden's botched withdrawal from Afghanistan that resulted in the deaths of 13 U.S. service members and 170 Afghans.

'I was getting out of Afghanistan, but we were getting out with dignity, with strength, with power,' Trump said. 'He got out, it was the most embarrassing day in the history of our country's life.'

On immigration, too, Trump was a clear winner again and again.

'I'd love to ask [Biden] why he allowed millions of people to come in here from prisons, jails, and mental institutions to come into our country and destroy our country.'

He predictably slammed Trump as a 'convicted felony,' but then became disturbing more specific.

Surveys routinely show that voters trust Trump over Biden on this issue, which ranks among their top concerns. After tonight, that poll margin will only grow wider.

And, of course, there is the issue of the economy – and persistent inflation eating away at American paychecks.

'[Biden] inherited almost no inflation,' Trump observed, 'and it stayed that way for 14 months. And then it blew up under his leadership because they spent money like a bunch of people that didn't know what they were doing.'

BIDEN CAN'T ANSWER MOST IMPORTANT QUESTION

CNN moderator Dana Bash saved the best question for last.

'President Biden, you would be 86 at the end of your second term. How do you address concerns about your capability to handle the toughest job in the world well into your 80s?' she asked.

The President's answer – in short – is not well.

'I spent half my career being -- being criticized being the youngest person in politics… And now I'm the oldest,' he replied, before launching into a meandering recitation of his record that included the invention of 'computer chips' and visits to South Korea.

Before Thursday night, most Americans already thought Biden couldn't serve another four years as president.

Now, surely, all of them, if they are being honest, will agree with that assessment.

Biden's candidacy is cooked.