A national progressive group, Indivisible, is asking the 20 candidates in the Democratic presidential race to sign a pledge promising a positive, “constructive” primary that ends with all participants coming together to support the eventual nominee — “whoever it is — period.”

The “2020 Candidate Pledge,” posted online on Tuesday, sprang out of discussions among prominent Democratic organizations about a unity pledge in the weeks after Bernie Sanders accused the “political establishment” of “plotting” to undermine his presidential campaign, reflecting a field that remains fractured after the bitter primary fight three years ago.

Drafts of a separate pledge, which uses slightly stronger language than the one released by Indivisible, are still circulating among officials from other groups, according to Democratic operatives.

The Indivisible document asks candidates to agree to three terms: “make the primary constructive” and “respect the other candidates”; “rally behind the winner”; and “do the work to beat” President Donald Trump. “Immediately after there’s a nominee, I’ll endorse,” the pledge reads.

As of Tuesday morning, no candidate had yet signed the pledge.

Sanders, the 77-year-old independent who challenged Hillary Clinton for the nomination in 2016, occupies a central spot in debates about the shape and feel of the 2020 primary.

He has promised to run a positive campaign and support the eventual nominee — telling voters that defeating Trump, a president he describes as an existential, unprecedented threat, is his top priority — yet he has said he will fight back against a “Democratic establishment” that would rather see his campaign fail. His top advisers, meanwhile, have said that, in a large field with multiple top-tier candidates, they believe they can win the nomination with just a third of the vote, setting up the possibility of a contentious convention with multiple rounds of voting by party delegates.

“Last time you had to get 50 plus 1 to win, and we started at zero,” said Ben Tulchin, who handles polling for the Sanders campaign, in an interview earlier this week. “Now we need basically about a third to win. And others have a lot farther to go than us to get to that third.”

Earlier this month, Sanders sent a pointed letter to the Center for American Progress, a prominent liberal think tank founded by Clinton’s former campaign chair, John Podesta. The group’s political arm owns a left-leaning news outlet, ThinkProgress, which published a story and attack ad–style video about Sanders’ newly accumulated wealth, prompting Sanders to accuse the think tank of sponsoring a “smear” campaign against progressives.

Over the next week, following a report in the New York Times about “Stop Sanders” Democrats, his campaign launched an “emergency” fundraising drive, warning supporters that the political establishment would seek to “derail our movement" and “defeat our political revolution.”

At the start of the race, the Democratic National Committee asked every candidate to sign its own pledge, which asked presidential contenders to self-identify as members of the Democratic Party, to “run as a Democrat,” and “serve as a Democrat if elected” to the White House.

Sanders, along with the rest of the field, signed on.