National progressive outfit Indivisible on Thursday launched a pledge compelling all 2020 Democratic presidential candidates to rally behind the eventual nominee. Bernie Sanders was the first to sign it, the group revealed.

The “We Are Indivisible” pledge asks for three commitments from him and any other prospective signers, as first reported by BuzzFeed News. First, Indivisible’s pledge requests that candidates “make the primary constructive” by outlining their visions while respecting their opponents.

“I’ll support the ultimate Democratic nominee, whoever it is—period," the pledge also reads. “No Monday morning quarterbacking. No third-party threats. Immediately after there’s a nominee, I’ll endorse.”

And the group also wants a commitment that every candidate will “do the work to beat Trump,” including a promise that “As soon as there is a nominee, I will put myself at the disposal of the campaign.”

“We believe in rigorous and spirited primaries, and we also know that once we have a nominee, our entire focus must turn to defeating Trump. The ‘We Are Indivisible’ Pledge commits all of us to a debate of ideas followed by dedicated work to make our ideas reality,” said Indivisible’s co-executive directors Leah Greenberg and Ezra Levin. “This pledge is about beating Donald Trump and the anti-democratic, xenophobic right wing. And it’s about the ideas and vision we need for a post-Trump future.”

Sanders has consistently pledged to run a positive campaign and to support any eventual nominee, even referring to some of the competitors in the race as friends. Even on Thursday, Sanders referred to the former Vice President Joe Biden—the latest and perhaps most formidable candidate to enter the race—as a friend with whom he has many disagreements.

However, Sanders has never been shy about criticizing more establishment-aligned elements of the Democratic Party. He recently sent a scathing letter to liberal think tank Center for American Progress criticizing its corporate contributions and content that was overtly critical of him for his wealth.