But while the establishment of the new group, Our Revolution, has been eagerly awaited by many of his most ardent supporters, it has been met with criticism and controversy over its financing and management.
A principal concern among backers of Mr. Sanders, whose condemnation of the campaign finance system was a pillar of his presidential bid, is that the group can draw from the same pool of “dark money” that Mr. Sanders condemned for lacking transparency.
The announcement of the group, which will be livestreamed Wednesday night, also comes as the majority of its staff resigned after the appointment last Monday of Jeff Weaver, Mr. Sanders’s former campaign manager, to lead the organization.
Several people familiar with the organization said eight core staff members have stepped down. The group’s entire organizing department quit this week, along with people working in digital and data positions.
After the resignations, Mr. Sanders spoke to some who had quit and asked them to reconsider, but the staff members refused.
At the heart of the issue, according to several people who left, was deep distrust of and frustration with Mr. Weaver, whom they accused of wasting money on television advertising during Mr. Sanders’s campaign; mismanaging campaign funds by failing to hire staff or effectively target voters; and creating a hostile work environment by threatening to criticize staff members if they quit.
Claire Sandberg, who was the organizing director at Our Revolution and had worked on Mr. Sanders’s campaign, said she and others were also concerned about the group’s tax status — as a 501(c)(4) organization it can collect large donations from anonymous sources — and that a focus by Mr. Weaver on television advertising meant that it would fail to reach many of the young voters who powered Mr. Sanders’s campaign and are best reached online.
“I left and others left because we were alarmed that Jeff would mismanage this organization as he mismanaged the campaign,” she said, expressing concern that Mr. Weaver would “betray its core purpose by accepting money from billionaires and not remaining grass-roots funded and plowing that billionaire cash into TV instead of investing it in building a genuine movement.”
Kenneth Pennington, who was the digital director of Our Revolution, declined to go into detail about his reasons for leaving but did confirm that he was no longer with the organization.
The staff members who quit also said that they feared that the 501(c)(4) designation meant the group would not be able to work directly with Mr. Sanders or the people he has encouraged to run for office because such organizations are not allowed to coordinate directly with candidates. Mr. Weaver did not respond to requests for comment.
In an email message sent to Sanders supporters on Tuesday night encouraging them to participate, Mr. Weaver said that the new group would work together to “empower a wave of progressive candidates this November and win the major upcoming fights for the values we share.”
Mr. Sanders has been using his vast list of supporters this summer to raise money for local lawmakers like Chris Pearson, a state representative in Vermont. He is also supporting candidates such as Tim Canova, a liberal Democrat who is trying to unseat Representative Debbie Wasserman Schultz of Florida, whom Mr. Sanders accused of favoring Hillary Clinton’s campaign during the primary race.
The creation of the group comes as Mr. Sanders faces lingering disappointment from some of his supporters for his endorsement of Mrs. Clinton and questions about his finances that have arisen since he left the race.
During the primary race, Mr. Sanders repeatedly delayed releasing his financial disclosure information, and ultimately never did. This month, he spent nearly $600,000 on a vacation home in Vermont near Lake Champlain.
Republicans in Mr. Sanders’s home state have taken notice of his post-campaign behavior and his new political organization and accused him of violating his own principles.
“He preaches transparency and then he tries to set up the most shadowy of shadowy fund-raising organization to support his causes,” said Brady Toensing, the vice chairman of the Vermont Republican Party, who argued that Mr. Sanders was essentially a big donor. “What I’m seeing here is a senator who is against big money in politics, but only when it’s not his big money,” he said.
Paul S. Ryan, a campaign finance expert at the Campaign Legal Center, a nonprofit political finance group, said that it was unusual for a federal lawmaker to set up such a fund-raising organization and that Mr. Sanders should have to follow the donation limits and disclosure requirements to be in compliance with the Federal Election Commission.
“There are definitely some red flags with respect to the formation of this group that are worth noting and keeping an eye on,” Mr. Ryan said. “We’re in a murky area.”