In this May 28, 2019, file photo, whitewater rafters paddle on the Kennebec River in The Forks. Central Maine Power's controversial hydropower transmission corridor plan initially called for high voltage power lines to cross over the Kennebec River Gorge. The new plan would tunnel under the river to preserve the views. Credit: Robert F. Bukaty | AP

AUGUSTA, Maine — Two groups opposing the November referendum aiming to kill Central Maine Power’s proposed hydropower corridor have spent nearly $9.5 million since last year, foreshadowing a fight that is on track to be the most expensive of its kind in Maine history.

A referendum aiming to block the corridor that would bring hydropower from Canada through western Maine is expected to be on the ballot in November after controversy about the notarization of signatures, though it is awaiting a final ruling by a Superior Court judge.

Clean Energy Matters, a PAC funded by Central Maine Power, spent more than $5 million during the first quarter of 2020, according to a report filed with the Maine Ethics Commission, after spending $2.3 million during the final months of 2019. A second group funded by the Canadian company Hydro-Quebec has an additional $2 million.

The bulk of that spending went toward online, TV and print advertising in the highest sum ever spent on one side of a Maine referendum campaign, exceeding the roughly $8.5 million spent by backers of an unsuccessful 2017 campaign to establish a York County casino.

A PAC supporting the referendum, Mainers for Local Power, spent just over $600,000 during the first three months of the year, the majority of which was paid toward Revolution Field Strategies, a Boston-based organizing and consulting group, for signature-gathering efforts.

Mainers for Local Power is funded entirely by Calpine and Vistra, two Texas-based energy companies that might lose revenue due to lower energy prices if the transmission project were completed. The group was fined $2,500 last month by the Maine Ethics Commission for spending to support the potential referendum before registering as a committee. The Hydro-Quebec committee paid a nearly $35,000 fine for similar reasons earlier this year.

A second group supporting the referendum, No CMP Corridor, spent only a few thousand dollars, but it did receive nearly $34,000 in in-kind contributions from Stop the Corridor, a limited liability company that is currently facing its own campaign finance investigation over allegations that its activities related to the referendum should require it to register as a committee and disclose its donors.

Leadership for Stop the Corridor has argued that it is attempting to influence the transmission project through the permitting process. The group ran TV ads about the corridor in January, but has not done so since. It briefly ran Facebook ads in March advertising a petition for the Maine Department of Environmental Protection to deny the corridor a permit.