Robyn Beck/AFP/GettyHOUSTON'After the third round of Democratic primary debates Thursday night, former Vice President Joe Biden emerged as he entered: the frontrunner. But it was survival. Not a win. Because over the course of three hours, and the post-debate spin as well, Biden's mental standing was not just the subtext, but the focus of the conversation.Some candidates attacked him without claiming they were doing so. Others were much more open about their motives.'There are definitely moments where you listen to Joe Biden and you just wonder,' Sen. Cory Booker (D-NJ) said soon after the debate concluded.In the end, it may not matter. Biden fended off attacks from his fellow candidates and left the night relatively unscathed. But the issue is now plainly out in the open as evidenced by his campaign's scramble to deal with it.'Attacking Joe Biden is not the way to advance yourself in the polls,' said Kate Bedingfield, Biden's deputy campaign manager and communications director. 'Candidates are seeing that and learning that lesson... our hope is that this will continue to be a more substantive conversation moving forward.'Symone Sanders, another Biden campaign senior adviser, audibly chuckled at the idea that voters might be concerned about broadsides attacking the former vice president's age, fortitude, and credibility among Democrats.Voters, Sanders told reporters after the debate's conclusion, 'know Joe Biden has shown up time after time, so asserting anything that flies in the face of what the voters know to be true themselves, because of their experience with Vice President [Biden], just won't stick.'Biden and his team had ample time to prepare for questions about his age, acuity, and stamina. His two previous debate performances had been criticized for their shakiness, and in the run-up to the third his staff insisted that he had put in the work to avoid a similar performance.Democrats: You're Still Stuck With Joe Biden for Now'If anyone is questioning whether he's up to the task, just watch him,' a senior Biden campaign official told reporters ahead of the debate. 'He has a big target on his back, and the other candidates have made clear in previous debates that they are going to be launching attacks on him,' the official added.Early on, Biden did seem up to the task. He sparred with the leading progressives in the race over their Medicare for All plans, holding his own against Sens. Elizabeth Warren (D-MA) and Bernie Sanders (I-VT) as they accused him of incrementalism in his approach.In the process, however, the attacks on his acuity began. Former Housing and Urban Development Secretary Julin Castro accused Biden of forgetting that he had said people would have to opt in to get Medicare-like insurance under his plan.'You just said that two minutes ago. You just said two minutes ago they would have to buy in,' Castro insisted. 'Are you forgetting what you said just two minutes ago''Biden hadn't actually done what he was accused of. Indeed, moments earlier, he'd said the opposite: 'Anyone who can't afford it gets automatically enrolled in the Medicare-type option we have.'But the seed had been planted by Castro, who would later claim that he was in no way trying to question the former VP's mental state.'No, not at all,' Castro told reporters after the debate, asked whether he was implying anything about Biden's age or mental acuity. 'I was going after the difference that we have in health-care policy, and what he just said on the stage.'From there, Biden offered up fodder for more questions. At one point, he seemed to conflate Afghanistan with Iraq, as he discussed how the former was actually three regions forced into one. At another point, he spoke about the educational benefits that playing records at night gave to underprivileged children, before pivoting to discuss Venezuela's Nicols Maduro. Biden said that non-violent criminals should not be in jail, only for his campaign to clarify that he was speaking of non-violent drug offenders.While several of these moments passed without commentary during the debate, privately Democratic operatives were buzzing with concern.For Biden's defenders, Castro's attack'and the questions it prompted'reeked of opportunism, and tacky opportunism at that.'It was unfortunate that Secretary Castro decided to go the way he did,' Biden campaign adviser Anita Dunn told reporters in the spin room following the debate's conclusion, noting a 'different attitude' towards both the former vice president and his tenure in the Obama administration than in debates past.And even some of Biden's opponents came to his defense. Sen. Amy Klobuchar (D-MN) called Castro's charge 'so personal and so unnecessary.'Biden's allies pointed to his lengthy history in politics as a way of stressing the voters would judge his record and not his debate moments.And Biden did not shy away from talking about that lengthy record on Thursday. In fact, he took pains to remind listeners of his time as a public defender while Dr. Martin Luther King Jr. was assassinated, about his role in the passage of the Brady bill in 1993, as well as his sponsorship of the Violence Against Women Act 25 years ago'all in response to a seemingly unrelated question about Obama-era deportations.What may benefit Biden the most is that the punches directed his way seemed to fall flat and others in the field declined to pile on.Biden Goes After Warren and Bernie From the Opening Bell'It certainly was uncomfortable,' said entrepreneur Andrew Yang when asked what he made of Castro's attack on Biden. 'There's probably a meme of my facial expression being like, that just happened'' Yang said, echoing South Bend Mayor Pete Buttigieg's onstage reaction that the exchange 'reminds everybody of what they cannot stand about Washington.''I probably reacted in the same way that most Americans did,' Yang said.Sen. Kamala Harris (D-CA), who blindsided Biden during the first debate in Miami, appeared less inclined to take a swing at the frontrunner.After moderator David Muir questioned Harris about whether her plan to ban the import of assault weapons through executive order was realistic, citing Biden's comments about the topic, Biden interjected, 'Some things you can. Many things you can't.'Instead of taking a swing, Harris opted for an awkward laugh-jab. 'Well, I mean, I would just say, hey, Joe, instead of saying, 'No, we can't,' let's say 'Yes, we can.''Biden, ignoring the overt reference to the Obama-Biden campaign slogan, responded, 'Let's be constitutional. We've got a Constitution.'Read more at The Daily Beast.Get our top stories in your inbox every day. Sign up now!Daily Beast Membership: Beast Inside goes deeper on the stories that matter to you. Learn more. Click here to read full news..