Julián Castro accused the former vice president of contradicting himself on whether people would need to opt into his Medicare plan, though it did not appear he had.
The first debate exchange captured the battle within the Democratic Party over how the presidential nominee should change the country’s health care system.
Some Democrats are proposing a government alternative to private insurance. But allowing people to choose such a plan may destabilize the A.C.A., some experts say.
Ms. Harris is forging her own path on the health care issue, putting forth a plan for providing Medicare for all Americans that stops short of the complete overhaul Bernie Sanders has proposed.
The so-called Cadillac tax on high-cost health plans, a key way the Affordable Care Act was supposed to pay for itself, was dealt a blow by an unlikely foe: Democrats.
Mr. Biden, detailing his health care plan in Iowa, paraphrased a notorious promise made by Mr. Obama: “If you like your private insurance, you can keep it.”