An uncomfortable topic that has lingered just below the surface of the primary for months erupted into clear view before millions of Americans on national television

HOUSTON - To many, it was the most loaded question at Thursday night's Democratic presidential debate.

"Are you forgetting what you said two minutes ago?" Julián Castro asked Joe Biden in a heated exchange over health care. The crowd inside the debate hall reacted with a collective gasp, as if they had witnessed a scandalous twist on an afternoon talk show.

Then, Castro repeated himself. Twice.

And just like that, an uncomfortable topic that has lingered just below the surface of the primary for months erupted into clear view before millions of Americans on national television: Are there issues with Biden's mental acuity and memory? Or not?

Castro's assertion that Biden, 76, contradicted what he had said earlier in the debate about his health-care plan didn't square with the former vice president's words, raising the possibility that Castro's aggressiveness could backfire.

The former vice president had just finished defending his plan to expand the Affordable Care Act with an optional public insurance program.

Biden said he was for an a optional public insurance program he dubbed "Medicare for Choice."

"Anyone who can't afford it gets automatically enrolled in the Medicare-type option we have," he said. Biden later said that people who lose their jobs and associated insurance automatically can buy into the plan. "You don't have - no preexisting condition can stop you from buying in. You get covered, period."

Castro attacked.

"The difference between what I support and what you support, Vice President Biden, is that you require them to opt in and I would not require them to opt in," he said. "They would automatically be enrolled. They wouldn't have a buy in."

Biden quickly shot back: "They do not have to buy in. They do not have to buy in."

Castro pressed on. "You just said that. You just said that two minutes ago. You just two minutes ago that they would have to buy in."

After more crosstalk, Castro delivered the line that would echo across the Democratic Party on Thursday night, asking Biden whether he was forgetting what he said two minutes earlier.

Biden turned to Sen. Bernie Sanders, I-Vt., who was standing to his right him on stage. The two spoke briefly. Then Castro attacked again.

"I'm fulfilling the legacy of Barack Obama, and you're not," he said, going after one of the pillars of Biden's campaign.

"That'll be a surprise to him," Biden snapped back.

Castro also said Biden's plan would leave 10 million Americans without health insurance. When Biden rolled out his health-care plan in July, he said it would cover 97 percent of Americans - leaving out about 10 million Americans. But Biden did not seem to contradict himself on the issue of an optional public insurance program.

Still, Castro's comments brought attention to a subject that holds the potential to further complicate Biden's campaign. While he leads in most polls, the former vice president has faced questions about gaffes, misstatements and his age, most recently after he told a moving war story in which many details were incorrect.

Although it was unclear whether Castro intended to broach that topic of Biden's mental fitness, it was widely seen that way. Some top Democrats swiftly registered their disapproval.

"I just thought this was not cool," Sen. Amy Klobuchar, D-Minn., told CNN. "I thought that was so personal and so unnecessary."

Former Chicago mayor Rahm Emanuel, who served as President Barack Obama's first White House chief of staff, said it was a bad move. "It's how he said it," said Emanuel, who said he felt it came across as "mean" and "angry."

Castro said after the debate that he was "fact-checking" Biden, pointing out that his insurance plan did leave some Americans without coverage and brushing off the criticism that he was too harsh.

"The vice president and I were having a disagreement about health-care policy," Castro said. "If you think that any of us are going to go off in October of 2020 against President [Donald] Trump, and that President Trump is going to be the nicest guy he can possibly be on that debate stage, he's not."

Another one of the candidates in the debate, Sen. Cory Booker, D-N.J., told CNN that Castro had legitimate concerns. "I think that we are at a tough point right now because there's a lot of people who are concerned about Joe Biden's ability to carry the ball all the way across the end line without fumbling," Booker said.

While brief, the clash amounted to one of the most memorable collisions of the first three debates, pitting two veterans of the Obama administration separated by three decades in age against each other in an argument over who was better equipped to build on the 44th president's legacy.

Castro, 44, the former Housing and Urban Development secretary under Obama, has been one of the most aggressive debaters in the field. He sparred with former Texas congressman Beto O'Rourke over immigration in June, and it was clear early in Thursday's debate that he had his sights set on Biden.

Castro has been polling in the low single digits. Biden is the leader in almost all public surveys. Some Democrats felt Castro had gone too far.

"@JulianCastro is once again on the attack. But with this 'your forgetting what you said two minutes ago' attack on @JoeBiden, accusing him of betraying spirit of @BarackObama on health care, Castro is throwing long with great risk," tweeted David Axelrod, a former Obama adviser.

Democratic strategist Mo Elleithee tweeted, "I think a lot of Democrats are going to react badly to Castro's "are you forgetting" line to Biden."

In an interview with CNN, former Michigan governor Jennifer Granholm called Castro's swipe at Biden a "low blow" and a "really bad move."

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