This post has been updated.

The Democratic presidential campaign of Bernie Sanders said that 17,000 labor union members across the country participated in a conference call Wednesday night meant to rally rank-and-file workers supportive of the Vermont senator.

During the call, participants were repeatedly asked to “push 1” to commit to be a volunteer for Sanders, who has emerged as a far more serious challenger to Hillary Rodham Clinton than most anyone expected. The number of participants on the call could not be independently verified, and the campaign did not immediately say how many volunteers it had enlisted.

Late Wednesday night, a campaign spokesman said that 70,000 people had participated in the call. That number was revised to 17,000 on Thursday morning.

The call comes as both major Democratic contenders are competing for the blessing of labor unions, a still-vibrant force in party politics in several of the early nominating states.

During brief remarks on the conference call, Sanders touted his support for raising the minimum wage to $15 an hour, launching a $1 trillion program to rebuild the country’s infrastructure and creating a single-payer “Medicare for all” health-care system.

“Don’t let anybody tell you that we’re radical, that we’re outside the mainstream,” said Sanders, a self-described democratic socialist. “We are the mainstream.”

The call also featured several Sanders supporters, including RoseAnn DeMoro, executive director of National Nurses United, the first nationwide union to back Sanders.

Clinton has drawn support from the American Federation of Teachers and the International Association of Machinists and Aerospace Workers and could have several more early endorsements on the way.

Several of those who participated in Sanders’s call urged rank-and-file members to tell the leadership of their unions to hold off on making an endorsement, given Sanders’s uptick in the polls in recent weeks.

Sanders has acknowledged that one of the challenges his campaign faces in harnessing the energy of supporters into more lasting roles. In late July, his campaign hosted more than 3,500 gatherings around the country on a single day in an effort enlist supporters and donors. More than 100,000 people were said to have participated.