Washington (CNN) Joe Biden stumbled into a potential campaign-defining moment Thursday that pitched his effort to win the Democratic nomination onto suddenly shaky ground.

Reeling from an attack by a much younger rival, Kamala Harris , the 76-year-old delivered an unfortunate soundbite at the Democratic debate that threatens to symbolize his biggest political liability.

"My time is up," he said, meaning to cut short one of his own answers but instead drawing attention to a question that haunts his campaign: Has his generation's time come and gone?

Harris' assault was breathtaking, appeared planned ahead of time and exposed a willingness to attack she had not previously shown in a campaign that started strong but has slumbered ever since.

The immediate impressions left after a debate are often not how things appear a few days later. So it may be that Biden's bad night does not end up being as damaging as it first appeared.

It's even possible that some voters might have been offended at the audacity of the attack by Harris on a man who has been a devoted servant of the Democratic Party for decades.

And Biden -- who served as vice president to the nation's first black President, Barack Obama -- enjoys strong support among African American voters, a crucial Democratic constituency.

But it was easily the most compelling and memorable moment of the young 2020 campaign so far and leaves the ex-vice president confronting deeply uncomfortable questions about his hopes.

Biden, on the ropes as he's rarely been in half a century of public life, snapped his head towards Harris in disbelief as she said, "Vice President Biden, I do not believe you are a racist..."

The California senator was seizing on a recent comment by Biden in which he highlighted his work with segregationist senators in the 1970s to show he could work with people he disagreed with.

Beloved by many Democrats after a lifetime overcoming personal tragedies, Biden appeared shocked and deeply hurt to be faulted on civil rights, an issue on which he's fought for decades.

His struggle to respond effectively to Harris - and his decision to invoke a states' rights argument with all its troubling racial connotations, only compounded the damage.

Biden has been trying to dispel the impression that he's been left behind by a young and diverse party that's changed under him.

As the frontrunner he was always going to be the biggest target -- and he took fire from other rivals, including Rep. Eric Swalwell of California.

Not all of his performance was as bad as his exchange with Harris. But debates are won and lost in the breakout moments that can come to define a candidacy.

New questions on Biden's hopes

Biden ended the night besieged by questions over whether he is sufficiently nimble and energetic to triumph in the grueling struggle to win the Democratic nomination.

His wobbly performance will also renew doubts about his claim that he's the strongest campaigner to take on Trump in November 2020.

The President, who has a merciless nose for political weakness, may have spotted Biden's vulnerability earlier than anyone.

He's spent recent weeks slamming Biden as "Sleepy" and making dark remarks about the mental state of a rival his aides say concerns Trump over his appeal in rust-belt states.

There's little doubt that Biden has the experience required of a President, and as a former vice president, he knows exactly the burdens that come with the Oval Office entail.

But inevitably, there will now be more questions among Democrats about the wisdom of nominating someone who would be the oldest President ever elected.

"I think age is a very legitimate issue," said David Axelrod, who saw the toll the office of the President can exact when he was a senior adviser to the Obama administration.

"This is the hardest job on the planet. And (it is) sensible to ask the question whether people who are nearing 80 years old when they enter office are prepared for it."

"That's why tests like this debate are important," Axelrod, now a CNN political commentator said.

Biden can look to history for comeback hints

While it was a bad night for Biden, it may not be terminal for his hopes of fulfilling his lifetime dream to be President.

He's not the first veteran politician to take a hammering in a debate. And he may be simply out of practice. He's not faced comparable stakes since his 2012 debate showdown with GOP vice presidential nominee Paul Ryan -- which he won handily.

In his previous presidential campaign in 2008, Biden was outsider throwing bombs at the frontrunners. Now the roles are reversed. And he clearly wasn't ready for the intensity of the fight.

In many ways, Biden performed like his old boss, Obama, who was roughed up by Mitt Romney, as presidents often are in their first reelection debate. His performance also recalled President Ronald Reagan, who appeared old and out of touch in his first debate against Democrat Walter Mondale in the 1984 election.

Each of those Presidents licked their wounds and bounced back, and won reelection. Biden now faces a similar assignment to get his campaign back on track in the next debate, to be hosted by CNN at the end of next month.

He will need to substantially raise his game. He was a shadow of the witty, vibrant politician who fired off zingers in his debates under intense scrutiny earlier in the 21st century.

To the victor, go the spoils

Harris's strong performance in Miami immediately established her as a potential rising star of the 2020 campaign. She joins Massachusetts Sen. Elizabeth Warren who was praised for her performance in the first of the debate double headers on Wednesday.

Early debates are often not decisive in selecting a party nominee and questions remains about Harris' staying power.

In a CNN town hall earlier in the campaign, for instance, she stumbled on health care, and has adopted confusing positions on Medicare for All -- perhaps the central issue in the Democratic race.

On Friday morning, she was left explaining her health care plan yet again after being one of only two candidates to raise their hand when they were asked "Who here would abolish their private health insurance in favorite of a government-run plan?" Harris said she believed the question was about whether she, personally, would give up her private plan in favor of Medicare and that she does not favor abolishing private insurance.

She will now face all the scrutiny that comes with a rising campaign.

But it was the political instincts she showed in turning to Biden and challenging his past positions on race that will mean this debate may be remembered for years to come.

She rebuked Biden for his recent comments about segregationist senators and said she had been personally hurt by his position on integrating schools in the 1970s.

"There was a little girl in California who was part of the second class to integrate her public schools and she was bussed to school every day," Harris said.

"And that little girl was me."

Biden hits back with prosecutor jab

Biden, monetarily stunned, protested, rightly that he had a strong record on civil rights - and said local authorities, not Washington, where he was serving as a senator, took decisions that affected Harris.

As she tried to cross-examine him, Biden threw a jab of his own saying, "I was a public defender. I didn't become a prosecutor," in a reference to Harris's former job sending people to jail.

"I've also argued very strongly that we, in fact, deal with the notion of denying people access to the ballot box," Biden added.

"I agree that everybody once they -- anyway, my time is up. I'm sorry," Biden said, trailing off.

Harris was not the only candidate to try to grab a headline by attacking Biden and emphasizing the generational gap.

Swalwell said in a CNN interview earlier Thursday that Biden's ideas were "staler than Donald Trump's."

And on the debate stage, he added, "I was 6 years old when a presidential candidate came to the California Democratic convention and said it's time to pass the torch to a new generation of Americans. That candidate was then-Sen. Joe Biden."

"He was right when he said that 32 years ago. He is still right today."

Biden didn't seem too put off by his 38-year-old colleague's impudence, replying "I'm holding onto that torch. I want to make that clear," he said.