In this May 28, 2019, file photo, a homemade sign is posted on a telephone pole in protest of Central Maine Power's controversial hydropower transmission corridor in Jackman. Credit: Robert F. Bukaty | AP

AUGUSTA, Maine — Maine’s ethics regulator unanimously ruled Friday that a group opposing the Central Maine Power corridor must disclose financial information to staff so the board can continue a probe into whether the group’s activities violated campaign finance laws.

It is the latest twist in an ongoing saga of ethics complaints and other legal volleys ahead of the scheduled November referendum on the CMP corridor, which would bring hydropower from Quebec through western Maine to connect with the New England energy grid.

The ethics commission’s Friday decision stems from a January ethics complaint by Clean Energy Matters, a CMP-backed political committee supporting the $1 billion corridor’s construction, against Stop the Corridor, a nonprofit that opposes it.

Stop the Corridor spent more than $1 million on TV and Facebook ads opposing the corridor as of earlier this year. The source of its funds are not known publicly because 501(c)(4) nonprofits are not required to disclose their donors as long as politics is not their primary purpose.

The CMP-affiliated committee alleged that Stop the Corridor had received and spent more than $1,500 on to influence a ballot question, which would require it to register as a political committee under state law. Stop the Corridor gave more than $85,000 in in-kind contributions to No CMP Corridor, one of the groups actively supporting the referendum.

But Stop the Corridor said it did not need to disclose its donors, as would be required of groups aiming to influence a ballot question, because it was trying to influence the corridor through the permitting process rather than the referendum.

The Friday decision by the ethics panel means that if the nonprofit is required to register as a political committee, it would be forced to reveal donors publicly and could pay a fine. If the commission determines no wrongdoing, the list of donors disclosed to commission staff would be kept private.

The referendum on the CMP corridor project will be on the Maine ballot in November after the state’s high court ruled earlier this month that a controversy about notaries did not invalidate the petition process.