(CNN) Joe Biden says he opposes fellow Democratic presidential contenders' push for a "Medicare for All" program because such an overhaul of the nation's health system could not coexist with the Affordable Care Act.

The former vice president, in an exclusive interview with CNN's Chris Cuomo airing on Monday, argued that former President Barack Obama's signature health care law, also known as Obamacare, should be expanded to allow people to buy into government-run coverage.

But he said that "starting over would be, I think, a sin."

"That's why I'm opposed to any Republican who wants to dismantle it or any Democrat who wants to dismantle it," he said. "The idea that you're going to come along and take the most significant thing that happened -- that any president has tried to do and that got done -- and dismantle it makes no sense to me."

Biden is the only one of the four top-polling Democratic presidential contenders not to support single-payer health insurance. A switch to single-payer coverage, under a plan proposed by Vermont Sen. Bernie Sanders, would require a tax increase, though supporters have said Americans would spend less on health care overall because they would no longer be required to pay insurance premiums, deductibles and copays.

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