Speaking to labor officials in Iowa this week, Bernie Sanders unveiled a new twist to his “Medicare for All” plan. His centrist Democratic rivals pounced, accusing the original champion of government-run health care of softening his signature policy in order to placate angry union members.

Nonsense, his campaign responded.

So what's the deal?

The new provision is wonkish, so bear with us here. But bottom line, it would offer advantages to workers who negotiated health plans through their unions during the four years the country moved to a universal, single-payer system.

Here's a breakdown of Sanders' controversial tweak and what it actually amounts to:

How would it work?

Under Sanders’ new plan, companies with union-negotiated health care coverage would have to renegotiate their workers’ contracts. The National Labor Relations Board would oversee and enforce the negotiations once Medicare for All becomes law. Any resulting health care savings from the single-payer system would be required to be returned to workers in the form of higher wages or more generous benefits.

Workers who don't belong to unions wouldn't qualify for that treatment.

This plan is a small part of a major, 1,900-word policy aimed at strengthening organized labor across the country that, among other things, calls for a dramatic expansion of collective bargaining.

Sanders’ proposal does not change anything in the Medicare for All legislation he unveiled in April, but adds requirements for the transition period that were not sketched out in the bill text.

Notably, the plan does not allow for more private health insurance. A critical part of the debate over Medicare for All has centered on the fact that Sanders’ bill would essentially abolish private insurance, and that remains the same under his new policy.

What did critics say?

Sanders’ rivals in the 2020 primary immediately slammed the new proposal, accusing him of backtracking from his hard-line opposition to private health insurance and characterizing it as an admission of the policy’s flaws.

“Bernie’s rewrite is the latest example of a Medicare for All candidate moving away from the plan in the face of the fact that it’s both bad policy and bad politics,” said Sen. Michael Bennet (D-Colo.). “Now that Senator Sanders is reversing, Senator Warren and the other candidates should reconsider Medicare for All.”

Campaign aides to Kamala Harris, who originally signed onto Sanders’ bill before backing away from the proposal and introducing her own health care plan, seized on the announcement as well.

“Bernie’s backtrack doesn’t solve the problems with his plan, it just lets everyone know that they’re there,” added John Delaney, one of the primary race’s loudest opponents of the single-payer plan.

Other critics on social media pointed out that Sanders’ proposal follows some union leaders and members expressing concerns about Medicare for All, including directly to Sanders on the campaign trail in Iowa. Joe Biden, who opposes Medicare for All, said this week that union workers “don’t have to give up” their private insurance under his health care plan because labor organizations “negotiated really hard” for those benefits.

How did Sanders’ campaign respond?

Sanders’ aides took to social media to aggressively push back. Warren Gunnels, a senior adviser to Sanders, said on Twitter that he “didn’t amend” the Medicare for All bill and called a Washington Post headline about the proposal — "Sen. Bernie Sanders changes how Medicare-for-all plan treats union contracts in face of opposition by organized labor" — “bullshit” and “bogus.” Another staffer said the plan simply provided “additional value to worker contracts.”

Sanders’ campaign told POLITICO that the question of whether companies or union members would receive the savings from eliminating private insurance was always one that needed to be answered — and the NLRB provides a way to direct those funds to organized workers. For non-union workers who lack contracts, they said, there’s no such mechanism.

“Organized workers sit down and negotiate an entire wage and benefits package, often including health care. Because at the end of the day both sides are just negotiating over the overall size of that aggregate package, the company has agreed to pay the workers a dollar amount of wages and benefits. Those agreements may be locked in for many years. That's really the key,” said Jeff Weaver, a top adviser to Sanders. “To lock in workers who made wage concessions for health care for years is not fair.”

Who’s right?

Sanders’ critics say the new policy is an admission the original plan was bad for unions. But the leaders of many major labor unions, including the SEIU, the American Federation of Teachers and the Association of Flight Attendants, say they support Medicare for All and believe their workers would benefit from free and comprehensive health coverage that they don’t have to bargain for.

Yet the new details the campaign released do nod to the anxieties about the proposal that some key players in the labor world have raised. Even some of those who say many union workers are paying too much now for too little health care have a fear of the unknown, and the additional incentives Sanders is offering could go a ways toward assuaging those concerns.

"Bernie Sanders understands collective bargaining," said Sara Nelson, president of the Association of Flight Attendants. "He's making sure union members can capture the value we've previously negotiated in health care as increased wages or other contractual gains."

The claims that Sanders' plan would allow more private insurance seems to stem from the fact that his new proposal lets union members negotiate for private health benefits as long as they don't duplicate what's covered under Medicare for All (which is, essentially, everything). But Sanders has permitted that all along.

What's the upshot?

The accusations that Sanders is flip-flopping are inaccurate. The new proposal doesn't alter anything in the original bill and private insurance will still be virtually eliminated — even if does give union members a big advantage under the plan compared to non-unions workers with private insurance.