In this Tuesday, May 28, 2019 photo a homemade sign is posted on a telephone pole in protest of Central Maine Power's controversial hydropower transmission corridor in Jackman, Maine. The power corridor would extend 53 miles from the Canadian border into Maine's north woods on land owned by CMP. Credit: Robert F. Bukaty / AP

Outside groups fighting over the future of the Central Maine Power corridor have spent more than $3.7 million on signature gathering and public relations since a referendum on the issue was deemed unconstitutional in August, as the fight over the controversial hydropower transmission line appears poised to spill over into 2021.

Anti-corridor groups are pushing for another referendum aimed at blocking the construction of the 145-mile transmission line that would bring energy from Canada through western Maine, while a CMP-affiliated political committee continues to fight against those efforts and attempt to improve public perception of the project.

Filings submitted to the state ethics board on Tuesday show the best funded group opposing the corridor, Mainers for Local Power, has spent $637,000 since mid-October and $1.5 million since the ballot question was ruled unconstitutional in August. The group is funded by Calpine, Vistra and NextEra, three out-of-state energy companies.

Mainers for Local Power resumed advertising in December, with at least $588,000 spent on producing and placing TV and radio ads. The group also paid more than $570,000 to Revolution Strategies, a D.C.-based consulting firm, in October for signature-gathering efforts.

The referendum for which corridor opponents are currently gathering signatures takes a different strategy than the first, aiming to require a two-thirds vote in the Legislature for the use of public land for transmission lines. The public face of the ballot petition drive is No CMP Corridor, a grassroots organization that has spent just $25,000 since mid-August, according to state filings, though Mainers for Local Power has provided significant financial might.

On the other side, the primary group supporting the corridor’s construction, Clean Energy Matters, has spent $742,000 since mid-October, according to its filings, and more than $2 million since the referendum was struck down in mid-August.

Clean Energy Matters, which is funded by CMP, has focused its spending on outreach and advertising. One recent ad highlighted connections between the signature drive and the energy companies that fund Mainers for Local Power, encouraging viewers not to sign the ballot question petition. The group also spent $45,500 on polling over the past few months.

The planned November referendum on the corridor was the most expensive referendum campaign in Maine’s history, with Clean Energy Matters and Hydro Quebec, a group affiliated with the Canadian energy company behind the project, spending more than $17 million combined before the question was struck down.

Proponents of the corridor have vastly outspent its opponents this year, although the gap was narrower this fall amid renewed signature-gathering efforts. Hydro Quebec backed off campaigning this fall, according to its most recent filings.