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Tuesday’s Democratic debates turned quickly into a fracas over Medicare for All, with Sen. Bernie Sanders and Sen. Elizabeth Warren fiercely defending their policy from an onslaught from the more moderate candidates onstage.

The debate built to a crescendo when Rep. Tim Ryan (D-Ohio) accused Sanders of not knowing what impact the bill would have on union workers.

“I do know. I wrote the damn bill!” Sanders snapped back.

The tense exchange came at the end of a free-for-all as a trio of moderate Democrats desperate for a big moment at the expense of the progressive duo went after Sanders’ Medicare for All plan, which would end private insurance and replace it with an expansive single-payer system.

Ryan, former Rep. John Delaney (D-Md.), former Colorado Gov. John Hickenlooper, and Montana Gov. Steve Bullock attacked the plan as destabilizing, unrealistic, politically risky and too expensive.

Some of those moderates didn’t even wait to get to questions to go after Sanders and Warren on the policy.

“We can go down the road that Senator Sanders and Senator Warren want to take us, which is with bad policies like Medicare for All, free everything, and impossible promises that will turn off independent voters and get Trump re-elected,” Delaney said in his opening statement.

“It took us decades of false starts to get the Affordable Care Act. So let's actually build on it — a public option, allowing anyone to buy in,” Bullock said after slamming his more liberal opponents for prioritizing “wish-list economics” over realistic proposals in his opening statement.

When Sanders was asked about Delaney’s criticism, he fired back simply, “You’re wrong,” getting laughs from the crowd.

Sanders said the plan “is comprehensive” and pointed out that Americans regularly lose their health insurance when they switch jobs or their employer changes policies.

He got strong backing from Warren, who has endorsed his Medicare for All plan.

“We are the Democrats,” she said. “We should stop using Republican talking points in order to talk with each other about how to best provide that healthcare."

Recent polls show that Americans are skeptical of moving to a Medicare for All plan that would end private insurance, as Sanders’ plan would do. But Democrats are closely divided over the issue, which has emerged as a central flashpoint in the Democratic primary and became the first big disagreement of Tuesday’s debate in Detroit.