In this May 2019 file photo, whitewater rafters paddle on the Kennebec River in Indian Stream Township, Maine, near the site of the proposed Central Maine Power corridor. Credit: Robert F. Bukaty / AP

Two company-funded groups opposing the November referendum on the Central Maine Power corridor have spent a combined $16.7 million through the end of June, according to state campaign finance reports.

A referendum aiming to block the transmission corridor that would bring hydropower from Canada through western Maine will be on the ballot in November, although Secretary of State Matt Dunlap has indicated that it may be unconstitutional, as backers have argued in court.

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While campaigns for and against the referendum have been marred by legal battles, pro-corridor groups have continued to spend big in an effort to persuade voters to approve the project, which supporters say would help bring down greenhouse gas emissions and lower energy costs.

Clean Energy Matters, a political committee primarily funded by CMP and its parent company Avangrid, spent more than $3 million between April 1 and June 30, according to a campaign finance filing submitted to the state ethics board Monday. The majority of that total was spent on advertising. In total, the committee has spent $10.5 million to oppose the referendum.

A second group supportive of the corridor, affiliated with the Canadian energy company Hydro-Quebec, spent $4.2 million over the past three months, bringing its total political spending to $6.2 million, although much of the recent spending involved paying off earlier debt.

The two groups had already topped the Maine record for the most spending on a single side of a referendum earlier this year, surpassing the roughly $8.5 million spent by backers of a failed 2017 casino referendum.

Spending in opposition of the corridor was comparatively muted over the last few months, although one of the groups that has been most vocal in opposition to the corridor, a 501(c)(4) nonprofit Stop the Corridor, is not registered as a political committee, so both its spending and sources of funding are mostly unknown.

Stop the Corridor has spent more than $1 million on TV ads opposing the corridor and gave $85,000 to the No CMP Corridor, a registered political committee supporting the referendum. It also made a contribution of an unknown amount to the Natural Resources Council of Maine.

The group is also being investigated by the state’s ethics board to determine whether it should be required to register as a political committee and disclose its donors. The nonprofit has countered that it is trying to influence the line through the permitting process, not the upcoming referendum. It sued the ethics panel last month in an attempt to stop the investigation.

Opponents of the referendum have cited potential negative environmental consequences of the construction of the corridor in western Maine, while pointing to general discontent with CMP stemming in part from billing issues over the last few years.

A third group opposing the construction of the corridor, Mainers for Local Power, is funded by two energy companies, Calpine and Vistra. The group spent a total of $41,000 during the past three months, with the bulk of that money going toward polling.