Opponents of Central Maine Power Co.’s proposed $1 billion transmission corridor through Maine’s western mountains have gathered enough valid signatures for a ballot measure to stop the project, state officials said Wednesday.

The referendum, if backed by voters in November, would order the Maine Public Utilities Commission to reverse its May finding that the 145-mile line, stretching from the Quebec border to Lewiston, is in the best interests of the state. But legal experts said the proposal to force that permission to be revoked is unprecedented and would challenge the independence of the PUC, probably prompting a legal battle.

The corridor project, formally known as New England Clean Energy Connect, would be built by CMP and its parent corporation, Avangrid. It’s designed to provide hydroelectric power from generators in Quebec to utility customers in Massachusetts. The project has received most of the government approvals it needs, but is still waiting on decisions by the state Department of Environmental Protection and the U.S. Army Corps of Engineers.

A year ago, Gov. Janet Mills threw her support behind the project after negotiating a package of benefits to the state worth about $258 million over 40 years that are designed to lower electric bills, advance clean-energy efforts and fund other public projects.

But opponents argue the project would harm the environment without providing tangible benefits to Mainers.

The Secretary of State’s Office said the backers of the measure gathered 69,714 valid signatures, more than the 63,067 required for the referendum question to move forward, even though 12,735 signatures were found to be invalid.

Next, the signatures will be scrutinized by a pro-corridor political action committee for 10 days to see if there are reasons to challenge the secretary of state’s finding. It would next head to the Legislature, where it would become law if lawmakers adopt it without changes or be sent back to the Secretary of State’s office to be transformed into a ballot question.

The Legislature typically rejects such measures so they can go on the ballot and the issue is left for the voters to decide.

The Secretary of State’s Office then proposes wording for the question and accepts public comment on its language. Once the wording is adopted, the measure goes to the ballot. In this case, that means it would go on the Nov. 3 general election ballot.

One of the chief proponents of the referendum said the decision by the Secretary of State’s office on the signatures represents a major step toward getting the issue before voters to decide.

“We’re really pleased that Maine voters will be able to stop the corridor from going forward,” said Sandi Howard, the director of Say No to NECEC.

Clean Energy Matters, a political action committee supporting the project, said late Wednesday that it would examine the signature-gathering process to make sure it complied with state law. The group said the process might have violated Maine laws on circulating referendum questions. For instance, the group said, the backers of the referendum might have used people who were paid to circulate petitions who were not registered to vote in the state or had not established residency in Maine.

“We have real concerns that those paid to organize signature gathering efforts were willing to operate in bad faith and engage in a pattern of behavior that includes illegal and unethical activity intended to undermine Maine’s electoral process,” said Jon Breed, the director of Clean Energy Matters. “The tactics of the paid political consultants who comprised the organization behind the signature-gathering effort warrant further scrutiny.”

In a brief statement issued Wednesday, the group didn’t provide specifics to back up its allegations.

Clean Energy Matters spent $2.2 million in the last three months of 2019, much of it on advertising, to support the project. The group is funded by CMP and Avangrid.

“Instead of spending millions to try and extract the profits they’d make for their shareholders, Central Maine Power and Hydro-Quebec should respect the fundamental right of voters to decide this issue at the ballot box,” Sue Ely, a lawyer with the Natural Resources Council of Maine, said in a statement. Hydro-Quebec is the government-owned utility in Canada that will provide the electricity that will be sent over the electric lines to connect with the New England grid in Lewiston.

Ely said a poll a year ago showed 65 percent of voters are opposed to the project and noted that more than two dozen towns along the route of the project have either voted to oppose the corridor or rescinded previous statements of support.

But Howard admitted that opponents “have some work to do” to counter the financial strength of the pro-corridor group, although she expressed confidence that voters would reject the project in November.

She also said that there’s nothing that opponents can do to stop work on the corridor from beginning this summer if CMP and Avangrid get the remaining permits needed and decide to go forward.

CMP already has taken some initial steps to begin construction, such as ordering thousands of wooden “mats” used to help move heavy equipment through the woods. In a conference call with stock analysts last week, Anthony Marone, Avangrid’s president and chief executive officer, said once the company gets the final permits, it will make a decision on whether to start work late this spring or in early summer.

“Our goal right now is everything is systems go to begin construction in Q3,” Marone said of third quarter – July through September. “We’re going to monitor things in Maine to make sure, to see how things are going and we’ll make a decision as to how much we intend to invest at that point in time.”

Howard said that would be a risky move for the company to take, knowing that the permits might be rescinded if the referendum is passed.

“There’s some real concern for the shareholders and the (stock) analysts about the huge financial risk” that would pose, she said.

And, “If they start (work), I think they’re going to infuriate Mainers even more,” she said.