Former Vice President Joe Biden appeared to take aim at two of his top rivals for the Democratic presidential nomination over their support for a government-run "Medicare-for-all" health care plan during a speech Wednesday to organized labor at the Iowa AFL-CIO.

The front-runner among 2020 Democrats and the only top-tier contender who opposes a single-payer “Medicare-for-all” system told the audience: “You’ve negotiated really hard for your benefits — with your union, with the employer. In my plan, you get to keep it. You don’t have to give it up.”

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Biden proposes protecting and enhancing the Affordable Care Act – better known as Obamacare – by adding a public option that would give people the ability to purchase a public health insurance option like Medicare.

The former vice president’s comments at the presidential candidates forum were an indirect jab at the Democratic Party’s two progressive standard-bearers – Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts – who are battling for second place in the polls right now, behind Biden. Both senators support a "Medicare-for-all" plan that would eventually eliminate private health insurance.

Sanders and Warren were heavily criticized during last month’s second round of Democratic presidential primary debates by some of the lower-tier candidates for their "Medicare-for-all" plans — with some of their rivals arguing that union members would lose their current health care coverage under a single-payer system.

One of those critics made the same case in Iowa on Wednesday.

Speaking prior to Biden, former Rep. John Delaney of Maryland highlighted that “a lot of workers in this country have worked real hard to get good health care, often through their union and many of them like it. And we should not be running on a policy of taking away that union health care which you've paid for and you've made sacrifices to get.”

Delaney, the son of a union electrician, emphasized that “for all of these policies, whether it's health care, whether it's economic policy, whether it's educational policy, or whether it's environmental policy, we have to put the worker first.”

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Sanders and Warren also spoke at the Iowa AFL-CIO forum, but neither specifically mentioned their "Medicare-for-all" proposals.

Both senators have pushed back at criticism of their "Medicare-for-all" plans as "Republican talking points."

Health care has been a top issue with Democratic primary voters and “Medicare-for-all” has been very popular with the progressive base of the party. Public opinion polling has indicated that a majority of Americans would support such a plan if it allowed them to choose between a government-run public plan and certified private options.