Bernie Sanders vigorously defended his signature “Medicare for All” proposal on Wednesday while hitting back at his critics who say his push amounts to both bad politics and bad policy.

The Vermont senator's speech came as the fight over how to best provide health care for Americans has become an animating focus of the race for the Democratic presidential nomination. The fight is pitting Sanders, with his push for a single payer system of health insurance, against former Vice President Joe Biden, who is embracing a so-called public option that would give people the option of retaining private coverage.

"The current debate over Medicare for All has nothing to do with health care. We are not in a debate about which health care system is working well or which is better," Sanders said at George Washington University. "What the debate that we are currently having in this campaign and all over this country has nothing to do with health care, but it has everything to do with the greed and profits of the health care industry."

Sanders also insisted that despite what some people may be hearing, coverage would actually increase under Medicare for All, not decrease. And he said that Medicare for All would end up reducing overall health care spending in the United States while lowering the number of uninsured and underinsured people.

"Medicare for All critics tell us that Americans just love their private insurance companies. You know what, I have never met one person that loves their insurance companies," Sanders said. "I have met many people that do love their doctors and their nurses, who have very good experiences in their hospitals, and what we do is to say you can go to those doctors, you can go to those hospitals, but you're not going to have to deal anymore with rip-off insurance companies."

Sanders also announced he plans to reject campaign donations from health insurance and pharmaceutical industry executives, lobbyists and political action committees, and he challenged other Democratic presidential candidates to do the same.

Slipping in some public polling and outraised by some of his 2020 opponents, Sanders has been increasingly willing to go on the offense against Biden on the issue of health care, though Sanders didn't mention the former vice president or any other candidate in his speech.

On Monday, Biden released a proposal to expand the Affordable Care Act , warning that it would be dangerous to eliminate it and replace it with Medicare for All. Biden’s proposal would create a public option that would allow people to sign up for a government-run health system like Medicare if they were unhappy with private insurance.

This week in Iowa, where Biden pitched his plan to voters, he cautioned that a sudden transition of tens of millions of people to Medicare for All was "a little risky."

"Medicare goes away as you know it. All the Medicare you have is gone. It's a new Medicare system," Biden said at a presidential candidate forum held by the AARP. "It may be as good, you may like it as well, it may or may not, but the transition of dropping 300 million people on a totally new plan, I think is a little risky at this point."

He also raised the prospect that some people could see temporary gaps in coverage, a notion that Sanders has bristled at and called "obviously absurd."

Nine years after the Affordable Care Act was passed under the Obama administration, Americans are still more likely to support than oppose the law, 48% to 30%, according to an April poll from The Associated Press-NORC Center for Public Affairs Research , though a sizable share doesn’t take either side. Views of a single-payer health care system, in which all Americans would get their insurance from one government plan, are mixed: More Americans favor than oppose single-payer, 42% to 31%, and an additional 25% say they hold neither opinion. The public is even more supportive of a government health insurance plan that can be bought instead of a private insurance plan: 53% express support, compared with just 17% who oppose; 29% are neither in favor nor opposed.

A central question in the debate over Medicare of All is how the cost of such a plan would be covered. Sanders has said taxes would increase on middle-income earners. California Sen. Kamala Harris, who supports Medicare for All, told CNN this week that she wasn't "prepared to engage" in such tax hikes.

"The rules have been written against the middle class and working families for far too long, and it is not necessary that they'd be taxed even more to achieve what is achievable, by recognizing that that they don't have to pay more to receive a benefit they deserve which is access to health care," he said.

The roaring health care debate also is likely to further draw in the full Democratic field. Beyond Harris, several other leading 2020 candidates, including Massachusetts Sen. Elizabeth Warren, support Medicare for All. However, some other Democratic hopefuls have warned that the party is moving too far left and have supported a more centrist approach.

Among them is Colorado Sen. Michael Bennet, who has been sharply critical of Medicare for All, instead preferring a public option for health coverage that could be included in the current structure of the Affordable Care Act.

In an interview with The Associated Press as he campaigned in Iowa, Bennet said that Democrats would "never unify around Medicare for All" but that plans like his, which would create a public option but allow people to keep their private health insurance, could bring Democrats together and notch wins in states like Colorado.

"If you're gonna stand up and commit the Democratic Party to taking away from 180 million people, you'd better be clear on what the nuances are because when you're running against Donald Trump, it's going to be too late," he said.

Associated Press writers Elana Schor and Hannah Fingerhut in Washington contributed to this report.