In a landslide vote — more than 75 percent of respondents — Mr. Sanders earned the backing of members of the group, which has quickly become politically influential since its founding in 2017.

Once a fledgling collection of college students, frustrated that Democrats and Republicans were not acting more quickly to curb climate change, Sunrise has grown to 318 chapters nationwide, with more than 10,000 members.

Sunrise will host an event on Jan. 12 with Mr. Sanders in Iowa City to formally announce the endorsement. “We believe a Bernie Sanders presidency would provide the best political terrain in which to engage in and ultimately win that struggle for the world we deserve,” Varshini Prakash, a founder and the executive director of the movement, said in a statement. “Senator Sanders has made it clear throughout his political career and in this campaign that he grasps the scale of the climate crisis, the urgency with which we must act to address it, and the opportunity we have in coming together to do so.”