Former Housing and Urban Development Secretary Julián Castro accused former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE of "forgetting" how he described his own health care plan during Thursday's debate and of not fulfilling former President Obama's legacy.

Castro also said his plan would cover more people than Biden's plan due to automatic enrollment.

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"The problem with your plan is that it leave 10 million people uncovered," Castro told Biden. "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. They would automatically be enrolled."

"They do not have to buy in," Biden responded.

"You just said that two minutes ago, you just said two minutes ago that they would have to buy in," Castro said as the men shouted over each other. "Are you forgetting what you said two minutes ago?"

Castro's line drew a big reaction from the crowd.

"I can't believe that you said two minutes ago that they had to buy in and now you're saying they don't have to ... you're forgetting that," he added.

"You're automatically enrolled," Biden said in response.

"If you lose your job for instance, his health care plan would not automatically enroll you. You would have to opt in. My health care plan would," Castro said. "I'm fulfilling the legacy of Barack Obama Barack Hussein ObamaDemocrats ramp up pressure on Lieberman to drop out of Georgia Senate race The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE, and you're not."

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

Minutes earlier, when describing his health care plan, Biden said that people "can join immediately."

Both men served in the Obama administration.

The exchange follows several recent gaffes by Biden.

He said in recent weeks that Bobby Kennedy and Martin Luther King were assassinated in the 1970s and that poor kids are just as bright as "white kids." It also follows questions about Biden's age, as he is one of the oldest candidates in the race.

Biden's team responded to the exchange by posting a quote in which Castro praised Biden years ago.