The chief political action committee supporting Central Maine Power Co.’s 145-mile transmission line project spent more than $5 million in the first quarter of this year to oppose a referendum that could kill it.

In a report filed with the Maine Ethics Commission, Clean Energy Matters said it has now spent nearly $7.5 million in all to oppose the referendum initiative.

Nearly all that money came from CMP, the utility that delivers electricity to southern and central Maine. CMP and its parent company, Avangrid, would build the $1 billion electricity corridor, which would carry power generated by hydroelectric facilities in the province of Quebec to Lewiston and then, through the New England power grid, to customers in Massachusetts.

Another group supporting the project, Hydro-Quebec Maine Partnership, has spent another $2 million opposing the referendum. That group is backed by Hydro-Quebec, a government-owned power company in Canada that would supply the electricity to Massachusetts.

Groups opposing the corridor have gathered enough signatures to put a referendum question on the November ballot aimed at requiring the Maine Public Utilities Commission to revoke its approval of the project.

Project opponents are being vastly outspent by Clean Energy Matters and the Hydro-Quebec group. Mainers for Local Power has raised $771,000 and spent $722,216 in the fourth quarter of 2019 and first quarter of this year. No CMP Corridor raised $152,000 and spent $42,852 during the same period.

None of the groups has much cash on hand except for Clean Energy Matters, which had about $300,000 unspent at the end of March. The organizations on both sides have spent most of their money on consultants and advertising. By November, it’s likely to be the costliest referendum campaign in Maine history.

The Natural Resources Council of Maine, one of the corridor project’s chief opponents, called the level of spending by CMP “stunning.”

“At this rate, CMP and Hydro-Quebec could spend more than $20 million by November in their effort to sell Mainers a project they don’t want,” said Pete Didisheim, advocacy director of NRCM. “This level of spending is unprecedented (and) deeply troubling during the current coronavirus pandemic and, frankly, obscene.”

Jon Breed, principal officer for Clean Energy Matters, said opponents have spread “false and misleading information” about the corridor project. He also pointed out that Mainers for Local Power is chiefly backed by Vista Power and Calpine, two electricity generating companies that could be hurt financially by the entry of an alternative power company into the New England market and therefore have an incentive to oppose the corridor project, Breed said.

“We’re committed to making sure we have the resources to push back on these groups’ false attacks, to correct the record and to do so in a transparent way,” Breed said.