Once again, health care took up a large chunk of a Democratic primary debate. Once again, there were fights over costs, coverage, and whether the party is growing too extreme.

But this time, all of the front-runners were on stage together, providing the first opportunity for all them to take direct aim at each other, and their vastly differing healthcare plans. It made for some heated exchanges, putting Medicare for All supporters on defense. But it also showed clearly that some candidates are cautious not to criticize others' proposals too harshly.

Thursday night's debate featured two stalwart supporters of the single-payer Medicare for All healthcare plan: Vermont Senator Bernie Sanders, who introduced the bill, and Elizabeth Warren, one of the bill's cosponsors. And from the start, former Vice President Joe Biden came out swinging at them.

"How are we going to pay for it? I want to hear tonight how that's happening," he said, making the case that his public option proposal would cost less.

One of the main arguments against Medicare for All is that it would mean much higher government spending. Supporters counter that it would lead to overall less healthcare spending than the current system. (Studies have been mixed on this question of whether costs would be higher or lower.)

What would change, they argue, is that the spending would be done by the government, with taxpayer dollars, rather than via copays and premiums, for example.

Warren drove at this point when moderator George Stephanopoulos asked about the potential for higher taxes for everyday Americans. In her answer, she did not give a flat yes or no — and avoided giving an endlessly soundbite-able answer about the potential for higher taxes for everyday Americans.

"Instead of paying premiums into insurance companies and then having insurance companies build their profits by saying no to coverage, we are going to do this by saying everyone is covered by Medicare for All, every health care provider is covered," she said. "And the only question here in terms of difference is where to send the bill."

There are also many Democrats, such as Minnesota Senator Amy Klobuchar, who stress that Medicare for All would virtually eliminate private insurance.

"While Bernie wrote the bill, I read the bill," she said. "And on page eight of the bill, it says that we will no longer have private insurance as we know it. And that means that 149 million Americans will no longer be able to have their current insurance in four years."

Polling has shown that many Americans don't understand that private insurance would largely disappear under Medicare for All, and that the idea of eliminating private insurance makes the plan much less popular.

Altogether, eight candidates on Thursday's stage support either a public option or some other sort of overhaul that would still maintain a substantial role for private insurance.

Polling shows that a public option is far more popular than single-payer healthcare. A July NPR PBS NewsHour Marist poll found that 90 percent of Democrats, and 70 percent of all adults, support a public option. Meanwhile, 64 percent of Democrats (and 41 percent of all adults) support Medicare for All.

That's less support, but it's still a majority of Democrats who support Sanders' single-payer plan, some of them passionately.

Perhaps with that in mind, even candidates who support other plans refrained from attacking the proposal — or its author — too hard.

For example, while Klobuchar slammed Medicare for All, she made sure to praise Sanders himself for working with her on trying to bring down prescription drug prices.

California Sen. Kamala Harris, who cosponsored Sanders' bill, sold her plan as a "Medicare for All plan," though it is substantially different from what Sanders has proposed. (It would, for example, retain a significant role for private insurance.) She also took care to praise Sanders, even while she supports a different plan.

"I want to give credit to Bernie," she said. "Take credit, Bernie. You know, you brought us this far in Medicare for All."

New Jersey Sen. Cory Booker, who also cosponsored Sanders' plan, said Thursday, "I believe in Medicare for All," but also proposed a more incremental approach. Indeed, in past debates, he did not indicate when asked that he would be willing to get rid of private insurance, which Sanders' bill would largely do.

In responding to Biden's attacks, Elizabeth Warren made sure to praise President Obama, saying, "we all owe huge debt to President Obama" for the Affordable Care Act. Likewise, Biden hugged Obama tightly in promoting his plan, saying that it would build on Obamacare.

Obamacare is popular among Democrats — 84 percent have a favorable view of it — and has grown in popularity among all Americans since Donald Trump's election. However, all Democrats in this field agree that it needs an overhaul. The balancing act that many are trying to do is pushing their own plans without slamming others too hard.

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