The warning is all but certain to set up a clash between Sanders’ most dedicated supporters and presumptive Democratic nominee Joe Biden, who opposes Medicare for All, at a time when the party is seeking to demonstrate unity ahead of its August convention. Though the petition signers have little chance of revising the platform to include Medicare for All support, they do have the numbers to draw attention to their protest and cause.

It is also likely to trigger fears among Democratic leaders about a repeat of the 2016 convention, when the party’s divides were on public display as Sanders supporters booed the mention of Hillary Clinton.

The left-wing groups Progressive Democrats of America and RootsAction.org are announcing Monday that they support the vow to vote against the platform if it doesn’t include Medicare for All. The Bernie Delegates Network, a coalition made up of hundreds of Sanders delegates, said it will also publicize the petition. Organizers expect those efforts will net hundreds more signatures, including from Biden delegates.

“The sea change that’s underway could swell as a result of this initiative,” said Norman Solomon, national director of RootsAction.org and a Sanders delegate. “It’s a reasonable hope that historians will look back at the next couple of weeks as a time when hundreds of delegates stepped forward and said, ‘This is a red line for a humane society and we’re not going to stop saying so.’”

Biden currently has 2,632 delegates, while Sanders has 1,076, according to POLITICO's delegate tracker.

Despite their frustration with Biden, progressive leaders behind the pledge all said they are voting for Biden and working to elect him. RootsAction.org, for instance, noted that it is planning to spend hundreds of thousands of dollars on an effort to persuade swing-state progressives who are on the fence to get behind Biden.

Organizers also said they are not being divisive — rather, it’s the party leaders who are overruling the grassroots who are being divisive, they argued. They point to exit polls showing that majorities of Democratic primary voters across states are in favor of Medicare for All.

"We're going to fight like hell for Biden. And there's no contradiction between doing that and supporting this pledge," Solomon said.

Whitmer said Sanders delegates in Nevada began circulating the pledge on Thursday through Slack and delegate groups. Though she did not provide precise numbers, she said it is safe to say that a majority who signed on are Sanders delegates, though some Biden delegates have as well. Activists hope a significant number of Biden delegates will support the vow.

The announcement of the pledge comes as President Donald Trump is attempting to paint Biden as a “puppet of the radical left.” His campaign has spent at least $14 million on a misleading ad claiming that police would be defunded under a Biden administration. The former vice president came out against defunding police almost as soon as it gained prominence.

Last week, a DNC committee released a draft platform that did not include support for Medicare for All or other top progressive priorities such as a jobs guarantee or the "Green New Deal," despite Sanders aides’ efforts to include them as planks. Organizers are going public with the Medicare for All pledge now in part because they hope to influence the Democratic National Committee’s platform committee, which is taking up the agenda on Monday. Whitmer said activists successfully persuaded a member of the panel to submit an amendment to back single-payer.

The pledge by delegates to vote against a platform without Medicare for All represents something of a split over strategy between Sanders and his most passionate supporters. After moderates accused him of failing to do everything he could to persuade his supporters to back Clinton in 2016 — a charge he vehemently denied — the Vermont senator has taken multiple steps to bring the Democratic Party together this cycle.

Sanders set up “unity” task forces with Biden, which released recommendations earlier this month. Progressives successfully pushed Biden’s allies to move left on certain issues, such as climate change, immigration and a no-deductible public option for health care. But they failed to persuade moderates on the task forces to back legalizing marijuana, end "qualified immunity," or embrace single-payer.

Sanders aides again tried to push centrists on the DNC’s draft committee to get behind Medicare for All, but failed to do so. However, they did manage to insert a nod to single-payer in the platform.

“Generations of Democrats have been united in the fight for universal health care. We are proud our party welcomes advocates who want to build on and strengthen the Affordable Care Act and those who support a Medicare for All approach; all are critical to ensuring that health care is a human right,” the draft states.

But some progressive activists were unimpressed, pointing to the more than 5 million people that liberal advocacy group Families USA said have lost their health insurance between February and May because of the coronavirus.

“Democrats who understand the profound need for Medicare for All don’t want a pat on the head,” Solomon said. “We want a genuine political commitment to health care as a human right. Biden hasn’t gotten there.”

Solomon and others said they have a different role than Sanders and the task forces, and vice versa.

In a Zoom call with his delegates last week, Sanders said of the task forces, “Did we get everything we wanted? Obviously we didn’t. That's for sure. But I think we reached agreements on all, in all of these areas."

He also urged his supporters to both “engage in coalition politics with the goal of defeating Trump,” and to "make sure that a Biden administration is the most progressive administration in modern American history."

Whitmer said she wants to make the party platform a more bottom-up process.

“In the Democratic Party, there’s an attitude that we have to write our platform to match the candidate and the candidate’s position,” she said. “We want to get back to the way it should be, which is the people create the platform based on our values and principles, and what we want the candidate to support.”