Hundreds of feet below a remote forest near Hudson Bay, Serge Abergel inspected the spinning turbines at the heart of the biggest subterranean power plant in the world, a massive facility that converts the water of the La Grande River into a current of renewable electricity strong enough to power a midsize city.

Mr. Abergel, a senior executive at Hydro Quebec, has for years been working on an ambitious effort to send electricity produced from the river down through the woods of northern Maine and on to Massachusetts, where it would help the state meet its climate goals.

Yet today, work on the $1 billion project is at a standstill.

Over the past few years, an unlikely coalition of residents, conservationists and Native Americans waged a rowdy campaign funded by rival energy companies to quash the effort. The opponents won a major victory in November, when Maine voters passed a measure that halted the project. Following a legal fight, proponents appealed to the state Supreme Court, which will hear arguments on the case on May 10 about whether such a referendum is legal.

At stake is more than one transmission line. The fiercely contested project is emblematic of fights going on around the country, as plans to build clean energy infrastructure run into opposition from residents resistant to new development, preservationists and other companies with their own economic interests at stake.

“At the end of the day, everyone might want more transmission for renewable energy,” said Timothy Fox, vice president at ClearView Energy Partners, an independent research firm. “But no one wants it in their backyard.”

The project in Maine, known as New England Clean Energy Connect, or NECEC, is the kind of large-scale, clean-energy infrastructure that will be required if the United States is to shift away from fossil fuels — a transition scientists say is urgently needed in order to prevent further catastrophic climate change. According to a major study by Princeton University, the country must triple its transmission capacity by 2050 to have a chance at reaching its goal of not adding any more carbon dioxide to the atmosphere by that point.

For years, everything in Maine was going according to plan.

State and federal regulators closely studied the project and gave approvals at every stage. Governors in Massachusetts and Maine were on board.

And Hydro Quebec and Avangrid, its partner on the project that will operate the transmission lines and equipment in the U.S., spent hundreds of millions of dollars readying construction and installing the first 78 of more than 832 new high-voltage transmission poles that would allow energy produced in northern Canada to keep the lights on in Boston.

But there was resistance to the project almost from the start. Maine residents, frustrated by years of poor service by Central Maine Power, a local utility owned by Avangrid, found common cause with environmental organizations skeptical of hydropower.

Those local groups found deep-pocketed supporters in three energy companies that operate natural gas and nuclear plants in the region and which stood to lose money if cheaper hydropower entered the New England grid.

After opponents got a referendum question about the project on last November’s ballot, both sides threw money at the issue, spending more than $100 million — a record for a Maine initiative — on a slugfest that tied the transmission project to hot button issues like gun rights and the Affordable Care Act.

Though Hydro Quebec and Avangrid outspent the opposition by a margin of 3 to 1, residents were not sold on the merits of the project. On Election Day, 59 percent of Maine voters approved a measure that brought work on the NECEC to a screeching halt, at least for the time being.

If the Maine Supreme Court sides with Hydro Quebec and Avangrid, work on the project could resume and electricity could be flowing from the reservoirs of Canada into the New England grid as soon as 2024.

But if the NECEC is scrapped, it will represent a major setback for those working to wean the United States off fossil fuels, according to independent energy experts. Development of a utility-scale clean energy project requires time and money, and the prospect that it could be killed by voters — even after it is vetted and permitted by government regulators — would inject a level of risk that could scare away investment.

“As hard as it is to explain and defend a project like this, it is so easy for people to come and torpedo it, and they don’t even have to tell the truth,” said Mr. Abergel. “If you can put a stop to these long term projects a year before they’re completed, it raises big questions about the energy transition and how we’re going to get it done.”

‘Rich with water’

Before there was a costly and acrimonious battle in Maine, there was a simple, idealistic mandate: Gov. Charlie Baker of Massachusetts, a Republican, wanted to reduce his state’s dependence on fossil fuels.

On a sunny Monday in August 2016, Mr. Baker appeared before the statehouse in Boston and signed a law intended to ramp up the use of renewable energy in Massachusetts. Hydroelectricity, he said, would “play a crucial role in the Commonwealth’s new balanced and diverse energy portfolio by offering clean, reliable and cost-effective base-load, 24/7/365.”

Mr. Baker’s focus on the always-on nature of hydroelectricity was intentional. While wind farms and solar panels can now produce substantial amounts of power, they cannot generate electricity when the air is still or the sun is not shining. But Massachusetts happens to be relatively close to one of the largest sources of clean, consistent energy in the world: Canadian hydropower.

Engineers have been tapping the Quebec region’s extensive network of rivers to produce renewable electricity for more than a century. Today, Hydro Quebec’s 61 hydropower plants produce 95 percent of all electricity in the province, and prices are lower than anywhere in the United States.

Hydro Quebec has also been exporting power to the United States and other Canadian provinces for decades. Five lines run from the company’s grid into New York, Vermont and Massachusetts, and another major transmission project is in the works to bring hydropower into the New York grid.

“We were blessed with a geology that is rich with water,” said Sophie Brochu, the company’s chief executive, sitting in her office in downtown Montreal. “The electricity is competitive and clean.”

So when Mr. Baker set a goal of drastically reducing Massachusetts’ emissions, Hydro Quebec seemed like an obvious choice.

And while Massachusetts was paying for the project, customers elsewhere, including in Maine, stood to benefit. Both states draw energy from the ISO New England power grid, a network of power plants and transmission lines that serves the northeast United States. Lower energy prices from hydropower would reduce costs for residents from Connecticut to Vermont.

By last year, work on the project was well underway. Hydro Quebec was clearing forest where it would install about 60 miles of transmission lines in Canada. Foliage had been cleared along most of the 145-mile-long transmission route through Maine. And in Lewiston, Maine, land had been prepared for a $330 million facility that would plug the electricity from Canada into the American grid, and deliver substantial tax revenues to the city.

Altogether, the project delivered what its backers believed was an unassailable combination of benefits. “This is an environmentally significant reduction in carbon emissions, and it also provides a huge amount of infrastructure that will enable new renewable generation,” said Thorn Dickinson, chief executive of NECEC. “You have the jobs, you have the property taxes, you have lower rates, all with no cost to Maine.”

‘A bad deal for Maine’

Many Mainers saw it differently.

Sandi Howard was rafting through a picturesque gorge on the Kennebec River in May 2018 when she first heard about plans to build transmission lines nearby. While much of the area around the river is crisscrossed with logging roads and cleared of trees, it is also a popular destination for rafters, snowmobilers and campers.

Ms. Howard soon emerged as one of the project’s leading antagonists. Armed with a Facebook group and a passion for the land, Ms. Howard spread the word about what she said was a fundamentally flawed project.

“As I started learning more, the concerns started to mushroom,” she said. “There’s a number of reasons why the project is simply a bad deal for Maine.”

Chief among Ms. Howard’s worries is the effect the new transmission poles will have on the local environment.

While roughly 100 miles of the new wire will be strung along an existing high transmission corridor that will be widened, the project will also require a cut through 53 miles of largely uninhabited forest near the Canadian border. Steel poles will be erected near streams where brook trout spawn, and in locations that could disrupt scenic vistas.

Those concerns, along with questions about whether the project would actually reduce greenhouse gas emissions, persuaded prominent environmental groups, including local Sierra Club and the Natural Resources Council of Maine, to oppose the project. Critics of hydropower contend that the large-scale flooding required to create reservoirs leads to emissions of methane, a potent planet warming gas.

And they say the overall climate benefits will be minimal because Hydro Quebec would not be generating new clean energy for the New England grid, just reducing the amount of hydropower it sells to other markets. A better solution would be the installation of rooftop solar across New England, the Natural Resources Council of Maine said, while other Maine residents point to what they say is a superior proposal to bring Canadian hydropower into the U.S. through an underground line in Vermont.

Native American tribes in Maine and Canada also joined the opposition, protesting the fact that corporations stood to “make billions of dollars in profits without consulting or compensating the First Nations on whose ancestral territories its electricity is produced and through which it will be transported.”

In a letter to President Biden, the Chief of the Penobscot Nation in Maine, Kirk Francis, said that, “the U.S. Army Corps of Engineers ignored its responsibility — and our requests — to consult with us and gave the NECEC its stamp of approval with blinders on.”

Yet another point of contention was the fact that many residents harbor deep animosity toward Central Maine Power and Avangrid. A history of poor customer service has made Central Maine Power one of the least popular utilities in the country, according to a study by J.D. Power.

As if all that were not enough, there was the fact that Avangrid is owned by a Spanish company, Iberdrola. That, along with Hydro Quebec’s involvement, led to claims that the project amounted to a foreign takeover of America’s energy infrastructure.

Before long, resistance had calcified, and many of the towns that initially voiced approval for the project began fighting it.

“I wanted to believe this project was a net benefit to the world with respect to climate, as well as a net benefit to Maine,” said Seth Berry, a representative in the Maine legislature and climate advocate. “But the more I looked into it, the more I realized it was neither.”

‘A lot of misinformation’

Though a diverse group opposed the plan, it wasn’t at all clear how they might stop a project that was already underway and had the support of senior state and federal officials. But Ms. Howard and her allies soon found well-funded partners that shared their agenda: three energy companies that operate natural gas and nuclear plants in the area and would likely take a hit to their profits if the NECEC project were to be completed.

The companies — NextEra Energy, Vistra Energy and Calpine — were soon funding a campaign to defeat the project, spending a total of $27 million on the effort, according to state filings.

Vistra and Calpine did not reply to requests for comment. NextEra said it was opposed to the NECEC for a variety of reasons, including the fact that completing it would require an expensive upgrade at one of its nuclear power plants in New Hampshire.

By last year, advertisements for and against the NECEC project were flooding the Maine media market, unleashing a dizzying series of claims and counterclaims that blurred the lines between fact and fiction. Battles raged over whether the project would result in overall greenhouse gas emissions, how severe the environmental effects would be, and how much Maine would benefit. Opponents of the project falsely claimed that hydroelectricity was dirtier than coal, while supporters tried to persuade voters that passing a retroactive law might one day jeopardize their gun rights.

The debates played out in town hall meetings, TV ads, direct mail and social media. The Fox News host Tucker Carlson, who has a home in Maine, produced a segment bashing the project. Energy Secretary Jennifer Granholm on Twitter touted the project’s potential to reduce carbon emissions and lower energy prices.

Hoping to win over skeptical Maine residents, Hydro Quebec and Avangrid modified the new transmission poles so they could also carry high speed internet cables, and offered the state a discounted rate on some energy.

It didn’t matter. On Election Day, Maine residents approved a carefully worded ballot measure that, if upheld by the state Supreme Court, will effectively kill the NECEC.

“This was the voters saying they don’t want projects like this in Maine,” said Tom Saviello, a former member of the State Senate, who became a leading voice of the opposition. “We were giving up a lot, and getting nothing out of this.”

But where Maine residents see a grass-roots victory, executives for Hydro Quebec and Avangrid, as well as Massachusetts officials, see a group of rival energy companies stymying the development of urgently needed clean energy infrastructure.

“The grid is going to have to get built out significantly to reach our decarbonization goals,” said Kathleen Theoharides, the Massachusetts secretary of energy and environmental affairs. “What makes me concerned is the idea that a project that was fully permitted by state entities could go to the ballot, and get a retroactive decision from the voters based on a lot of misinformation from energy companies that stood to lose money from this new line coming through.”

‘Not good for climate’

After a day spent touring the generating station in Radisson, Mr. Abergel boarded a small turboprop plane for a three-hour flight south to Montreal and reflected on a project that appears on the verge of collapse. From the air, he looked out on hundreds of square miles of uninhabited land, much of which had been flooded decades ago to create the massive reservoirs that power Hydro Quebec’s subterranean turbines.

“The project would give people a stable source of power — not to mention it’s clean,” he said. “Even if you don’t care about the environment, it makes sense.”

Yet as the Maine Supreme Court decides the fate of the NECEC, it will not be evaluating the project on its relative merits, or considering the swirling claims and counterclaims. Instead, the court will decide a narrow set of questions that have nothing to do with climate change, focusing on technicalities such as whether a referendum can stop a project that was already approved by regulators.

“This basically sets the precedent that voters can block these really important infrastructure projects,” said Robin Millican, director of policy at Breakthrough Energy, a group that is promoting various efforts to reduce emissions but is not involved in the project. “That’s not good for climate overall.”

Many analysts, and even supporters of the project, acknowledge that the court could side with the opposition, dooming the NECEC and forcing Massachusetts back to the drawing board. That is a scenario that would cost Hydro Quebec and Avangrid a small fortune, and could have far-reaching implications, spelling trouble for other efforts to rapidly deploy more clean energy across the country.