A group of Bernie Sanders supporters who have worked or volunteered for his campaign have come up with a draft proposal calling for the Vermont senator to suspend his presidential bid after the June 7 California primary, eventually concede to Hillary Clinton and build an independent organization aimed at defeating Donald J. Trump.

The three-page document addresses a question now facing Mr. Sanders’s most ardent supporters: how to harness the energy of a movement now that his presidential bid appears to be bowing to the reality that Mrs. Clinton will capture the Democratic nomination. The document suggests building “an organization, completely independent of the Clinton campaign,” to defeat Mr. Trump before focusing on other goals after the November election.

About a dozen current and former Sanders staff members and volunteers have been collaborating on the draft proposal, which was obtained by The New York Times and first reported by Politico.

While the authors believe Mr. Sanders will win the California primary, they write that “barring a miracle,” he will not become the party’s nominee. Two of the authors, who are former campaign staff members, confirmed the authenticity of the document. The authors all spoke on the condition of anonymity because they were working without the campaign’s authorization.