"Medicare for All," the progressive proposal that would rely on a single government entity to provide health care coverage to all Americans, has materialized as a defining issue of the 2020 Democratic presidential primary race at a time when its most vocal champion, Sen. Bernie Sanders of Vermont , is searching for a lever to reverse his discernible polling slide.

Joe Biden's release of a health care plan that is chiefly designed to protect and gradually build upon former President Barack Obama's Affordable Care Act with a public option – rather than push for a more ambitious single-payer system – has set off an intense debate with Sanders , who will devote an entire speech to the topic on Wednesday in Washington.

It also comes at a time when 19 Democratic presidential candidates are parading through an AARP forum this week in Iowa, where they're being probed for their views on the best mechanisms to improve and grow health care access before voters who will participate in the first nominating contest.

For the former vice president and primary race front-runner, Medicare for All is an unrealistic gambit that could endanger the entire system due to its enormous $30 trillion price tag. Using some of his most dramatic rhetoric to date, Biden on Monday in Des Moines said, "To add another roughly 300 million people in one fell swoop, understand … Medicare goes away as you know it. All the Medicare you have is gone."

Peering into the AARP audience, Biden asked how many people liked their employer-based health care. A smattering of hands sprung up.

"Now if I come along and say, 'Finished, you can't have it anymore.' Well, that's what Medicare for All does. You can not have it. Period," he told them.

His answers set off a furious reaction from Sanders and his legion of supporters, who accused Biden of mischaracterizing the impact of a four-year transition to Medicare for All. Sanders' campaign manager, Faiz Shakir, described Biden's public option as a "policy and moral failure" because it would still be too expensive for the neediest to afford and leave nearly 10 million uninsured.

The Sanders campaign even launched an online quiz asking participants to identify who has made various claims against Medicare for All, listing Biden, President Donald Trump and Senate Majority Leader Mitch McConnell as options, and arguing that Biden's "lies" are "straight out of the playbook" of Republicans.

"I've helped write and defended the Affordable Care Act," Sanders said Tuesday during a livestreamed interview with The Washington Post. "But you know what? Times change and we have got to go further."

"When Joe says something to the effect that Medicare for seniors … will end? That's just an obviously absurd situation," Sanders said.

Whereas Biden is pointedly framing his plan foremost as a gradual extension of Obamacare, Sanders has begun labeling his opponent's proposal "Bidencare," as a way to distinguish it from the former president's enduring popularity with Democrats.

Mark Longabaugh, a former Sanders adviser who split from the 2020 campaign earlier this year, notes that while both Biden and Sanders have established clear positions on Medicare for All, others, like Sen. Kamala Harris of California and Sen. Cory Booker of New Jersey risk damaging themselves if they fail to provide clarity on such a predominant issue.

"The folks that I say are not going to win this debate are those who tend to waffle on it," Longabaugh says. "Harris has twice … said she was for it, then came out of it and backtracked on it. It's really unclear to me what her position is at all. It basically sounds like Booker has said 'I signed the bill for aspirational reasons.' You can't have it both ways. You've got to make a decision on this bill."

Booker, appearing in Iowa on Monday, positioned himself as a supporter of Medicare for All, while acknowledging it was unrealistic to expect such a massive immediate transformation of the system. In theory, he sounded like Sanders, but in practice he echoed Biden.

"The culinary union in Nevada said to me, 'We don't like Medicare for All because we negotiated for our benefits. We like the health care that we're getting,'" Booker said. "Before we take tens of millions of people off of the health insurance that they like, let's show that we can create a viable strong public option for all Americans and improve Medicare from where it is now."

Responding to a question at the AARP traveling forum in Bettendorf, Iowa, on Tuesday, Harris said her vision of Medicare for All would essentially phase out private insurance over time, with the exception of supplemental insurance for procedures not covered in the legislation.

"There will eventually not be a need for private insurance because there will be full coverage in terms of most of the services folks need," she said.

Other more moderate candidates have consistently said Medicare for All goes too far in its elimination of private insurance and instead favored the public option path, which even President Obama failed to achieve.

John Hickenlooper, the former governor of Colorado, said Medicare for All "would be an evolution, not a revolution, and would take 10 to 15 years" while Sen. Amy Klobuchar of Minnesota said the U.S. would always have some private insurance as part of its health care system. Sen. Michael Bennet of Colorado even asked Sens. Sanders, Harris and Elizabeth Warren of Massachusetts to reconsider their support for the legislation.

A July CNN poll shows why Medicare for All is dicey territory for Democrats. Among all adults, just 21 percent said they favored national health insurance that would completely replace private health insurance. Among potential Democrats, that percentage grew to only 30 percent.

But as Sanders position flags in national and early state polling, he clearly sees the elevation of his signature issue as a way to resurrect himself as the true progressive in the primary race.