In this Tuesday, May 28, 2019 photo a sign in protest of Central Maine Power's controversial hydropower transmission corridor is stretched across a business sign in The Forks, Maine. The power corridor would extend 53 miles from the Canadian border into Maine's north woods. Credit: Robert F. Bukaty | AP

AUGUSTA, Maine — A dark-money group that has aired ads opposing Central Maine Power’s proposed hydropower corridor was hit with a Friday ethics complaint by a political committee funded by the utility in a move that could force the group to disclose donors.

It’s the second ethics complaint in the nascent campaign over the $1 billion corridor that would take Hydro-Quebec power to the regional grid with a 145-mile transmission line through western Maine as opponents collect signatures to put a question on the 2020 ballot to kill the project, which has won two required permits from regulators and awaits others.

During the final three months of 2019, CMP dumped $2.3 million into a political committee opposing the referendum bid. On Friday, that committee, Clean Energy Matters, filed a complaint with the Maine Ethics Commission against Stop the Corridor, a nonprofit that has not disclosed donors and has run $1.3 million in TV and Facebook ads against the project.

Filings with the commission indicate that Stop the Corridor gave nearly $50,000 in in-kind contributions to a political committee run by project opponents. That committee, No CMP Corridor, raised more than $26,000 in other contributions by December’s end.

The complaint alleges Stop the Corridor violated Maine law that makes entities register as political committees if they raise or spend more than $1,500 in a calendar year to influence a political campaign and that is its major purpose. If the nonprofit is forced to register as a political committee, it would have to disclose past donors and it could pay a fine.

Newell Augur, a lawyer for Clean Energy Matters, said in the complaint that Stop the Corridor and affiliated groups are “continuing to flaunt Maine election law by failing to report the actual amount and source of their funding.” Riley Ploch, an operative who heads Stop the Corridor, said in a statement the group believes it has “followed Maine’s disclosure requirements.”

Jonathan Wayne, the executive director of the ethics commission, said he expects commissioners to consider the complaint in late February or early March.

The other ethics complaint in the race was filed earlier this month by a legislator against a political committee run by Hydro-Quebec in opposition to the proposed referendum. That group disclosed nearly $100,000 in campaign spending before filing as a political committee.

Michael Shepherd joined the Bangor Daily News in 2015 after time at the Kennebec Journal. He lives in Augusta, graduated from the University of Maine in 2012 and has a master's degree from the University...