CONCORD, N.H. — In early December, about 100 activists aligned with Sen. Bernie Sanders (I-Vt.) gathered in South Bend, Ind., the home turf of rival candidate Pete Buttigieg. They held up signs touting Medicare-for-all and other signature Sanders proposals. One accused Buttigieg of neglecting African Americans. Another said it was crucial for Sanders to win the election.

This was not a Sanders campaign event. It was a rally staged by Our Revolution, a nonprofit founded by the senator that has caused some awkwardness for him in his second run for president. While the group is supporting his candidacy, it accepts large donations without fully disclosing who made them, a practice at odds with his calls for greater transparency and stated desire to curtail the power of the wealthy in elections.

Sanders was questioned about the group’s practices here Sunday, when he said in a radio interview he would “have no problem” with the group opting to provide more information about its donors. But he suggested he would not call on its leaders to do so while his opponents continue to rely on similar organizations.

"You've got groups all over the country that legally can do what they want. And I would be very happy to say and to urge an end to all that if other candidates do the same. So I am not in favor of these things," Sanders said in an hour-long interview on New Hampshire Public Radio. "That is the way it is."

These were his most detailed comments of the race on Our Revolution, an organization Sanders launched in 2016 before separating from the group. Though barred by federal laws from coordinating with his campaign, the group has emerged as a potent part of Sanders’s grass-roots army, with hundreds of thousands of members and activists working across the country to promote his presidential bid.

Tax filings show Our Revolution raised nearly $2.7 million in 2018, the most recent year for which records are available. That total includes more than $500,000 from 15 donors whose identities have been shielded. Two of those donors made six-figure contributions.

The scale of the group’s activities is small compared to other politically active nonprofits that have spent tens of millions of dollars trying to influence the election. And the money it collected in 2018 is dwarfed by the tens of millions Sanders and his rivals have hauled into their official campaign accounts.

Still, the group’s work shows how Sanders is benefiting from some of the types of donors he criticizes, as well as the kind of shadowy group he has decried.

Paco Fabian, a spokesman for Our Revolution, said the group follows all campaign finance rules and in some cases exceeds federal transparency requirements. But the group declined to identify its largest donors, saying it is respecting their wishes to remain private.

The Democratic primary has featured a vigorous debate about the role of money in politics. Buttigieg, the former South Bend mayor, has faced heat for raising money from rich patrons at a luxe wine cave in California. Former vice president Joe Biden has been chastised for benefiting from a super PAC launched by supporters that has no limits on what it can raise and spend. And Sen. Elizabeth Warren (D-Mass.) has faced intense scrutiny over her past willingness to cater to rich donors, even as she eschews it now.

Sanders supports legislation that would end political spending by politically active nonprofits “and other organizations who accept unlimited contributions or do not disclose donors,” according to his 2020 campaign platform. And he has frequently denigrated his opponents’ fundraising practices, lambasting those who have courted wealthy donors.

Sanders is a vocal critic of super PACs, which raise and spend unlimited amounts of money but are required to disclose donors. Our Revolution is not a super PAC and is not subject to the same donor disclosure requirements.

“I think we should end super PACs right now. So I would tell my opponents who have a super PAC, ‘Why don’t you end it?’ And certainly that’s applicable to the groups that are supporting me,” Sanders said in the Sunday interview.

Over the past year, Our Revolution has hosted Iowa town halls, organized post card writing in early states in support of Sanders’s candidacy and gone knocking on doors. The South Bend rally was one of its highest-profile events.

The group raises money through email appeals, not pricey fundraisers, and largely through small contributions, said Fabian, noting its average donation in 2018 was about $20. Less than 10 percent of the group’s work is electoral, according to Fabian. The issues it focuses on overlap heavily with Sanders’s platform.

Federal law requires nonprofits to disclose large donations — though not donor names — in tax filings, which Our Revolution does. The group also lists on its website the names of donors who gave more than $250. But it does not provide specific amounts the donors gave, and contributors who wish to remain anonymous are obscured — for example, a donor whose name is listed as “Anonymous Anonymous.”

“Voters don’t get the information they deserve, simply as a result of a group creating its own disclosure regime it imposes on itself,” said Paul S. Ryan, vice president of policy and litigation at Common Cause.

Sometimes, the line between Our Revolution and Sanders has blurred, such as when Sanders participated in a conference call with the group’s members.

And some of Sanders’s allies have shown up on the campaign’s rolls as well as the group’s leadership. Among them is Nina Turner, a national co-chair of the Sanders campaign who was paid nearly $200,000 by Our Revolution and served as president before stepping down to join the campaign.

Lee reported from Washington.