Castro sharply accused the former vice president of forgetting what he'd said just minutes before during a disagreement over a minor point in the candidates' broader discussion of whether to embrace "Medicare for All," which Biden has opposed.

"Are you forgetting what you said two minutes ago?" Castro, a former housing and urban development secretary, said to Biden. "Are you forgetting what you said two minutes ago? I can't believe that you said -- two minutes ago -- that they had to buy in and now you're forgetting that. We need a health care system where you're automatically enrolled."

South Bend, Indiana, Mayor Pete Buttigieg cut in, saying, "This is why presidential debates are becoming unwatchable."

He continued: "This reminds everybody of what they cannot stand about Washington. Scoring points against each other, poking at each other."

Castro replied, "That's called the Democratic primary election, Pete. That's called an election."

Sen. Amy Klobuchar of Minnesota then responded, "But a House divided cannot stand. And that is not how we're going to win this election."

Castro told CNN's Chris Cuomo after the debate that the comment was "not intended as a personal attack or affront."

"This is a debate. And when we're talking about health care policy, we're talking about a policy that impacts every single person in this country," Castro said.

Castro added on Friday that he would've asked the same question of any of the other candidates had they denied a comment they made within minutes.

"It didn't matter that it was Joe Biden. If it had been another candidate that tried to deny what they just said two minutes ago, I would have asked them the same thing," Castro said on CNN's "New Day."

In interviews with CNN immediately following the debate, Democratic candidates expressed disapproval of Castro's tactic and stressed the importance of party unity.

Klobuchar said Castro's attack was "not cool," and "so personal and so unnecessary." She said, "It feels like something that Donald Trump might tweet."

Sen. Bernie Sanders of Vermont said he is going to continue to contrast his views and record with Biden's, but that "I'm not going to go after him personally. That's not right."

Buttigieg said it was "starting to feel like a scrum up there."

Sen. Cory Booker of New Jersey said he is going to stand up against any kind of "demeaning and degrading between candidates," adding, "I've seen this movie before in the last election."

CNN's Dana Bash pointed out that Booker had been tough on Biden in the last Democratic debate , where the senator delivered one of the most forceful denunciations of Biden's record and suggested he former vice president had been one of the chief architects of a criminal justice system that disproportionately incarcerated black and brown men. Booker responded, "Absolutely, but you saw the way I did it. It wasn't a cheap shot."

Castro has "some really legitimate concerns" that are shared by many others, he said, about Biden's "ability to carry the ball all the way across the end line without fumbling." Booker said Castro has "every right to call that out."

But the senator stressed the importance of tone and tenor and disagreeing respectfully.

When you demonize somebody and create bad blood it's hard to unify afterwards," he said.

Sen. Kamala Harris, whose confrontation of Biden in the first debate earned her a boost in the polls, said the focus should instead "be on what we need to do to prosecute the case against Donald Trump."

"It's a debate stage. Things get heated," the California senator said Friday on CNN's "New Day," adding that "the focus needs to be on the fact that Donald Trump is not doing the American people the benefit of the office."