“Who here would abolish their private health insurance in favor of a government-run plan?” Holt asked.

The only two candidates who raised their hands were New York City Mayor Bill de Blasio and Sen. Elizabeth Warren, D-Mass.

De Blasio argued that private health insurance is not working for tens of millions of Americans, because of high premiums and high out-of-pocket costs.

“How can you defend a system that's not working?” de Blasio asked.

Warren accused health insurers of sucking $23 billion in profits out of the health care system in 2018.

“That doesn't count the money that was paid to executives, or the money that was spent on lobbying in Washington,” Warren said. “We have a giant industry that wants our health care system to stay the way it is. It's not working for families, but it's sure as heck working for them.”

Anonymous tweeters who said they support the Medicare 4 All proposals contended that Warren had decided to support Bernie Sanders' Medicare for All proposal only recently and questioned how firm her commitment to the proposal really is.

Sanders' Medicare for All bill calls for replacing private major medical insurance with a government health care program. The bill would also replace or crowd out private dental insurance, private vision insurance, private long-term care insurance, and the forms of supplemental health insurance designed mainly to pay patients' out-of-pocket medical bills.

Some of the other candidates in the debate said they support Americans' right to continue to choose to buy private health insurance.

John Delaney, who has represented Maryland in the House, said 100 million Americans say they like their private health coverage.

“I think we should be the party that keeps what's working and fixes what's broken,” Delaney said. “We should give everyone in this country health care as a basic human right for free, full stop. But we should also give them the option to buy private insurance. Why do we have to stand up for taking away something from the people?”

Delaney said his father was an electrician in the IBEW union.

“He loved the health care that the IBEW gave him,” Delaney said.

Delaney said advocates of the major Medicare for All proposals are assuming that a new federal health program could pay reimbursement rates based on the current Medicare reimbursement rates.

“If you go to every hospital in this country and you ask them one question, which is, 'How would it have been for you last year if every one of your bills were paid at the Medicare rate?,'  every single hospital administrator would say they would close,” Delaney said. “And the Medicare for All bill requires payments to stay at current Medicare rates. So, to some extent, we're basically supporting a bill that will have every hospital close.”

Sen. Amy Klobuchar, D-Minn., said she'd prefer to strengthen the current ACA system and, possibly, offer a government-run “public option” plan, without going ahead with the kind of elimination of private health insurance proposed in the major Medicare for All bills.

“I am just simply concerned about kicking half of America off of their health insurance in a four years,” Klobuchar said.

Read more: 

• Single-payer vs. Medicare for All: What's the difference?
• Medicare for All push is losing steam
• CBO: Medicare for All a 'major undertaking'