PORTLAND, Maine — The battle over a $1 billion green energy transmission line that won all regulatory approvals only to be rebuked by Maine residents in a retroactive referendum is going to a jury.

In a rare move, a jury of nine is being asked to decide a complicated constitutional matter — whether developers have a vested right to complete the 145-mile project, which would supply Canadian hydropower to the New England power grid.

The constitutionality of the statewide referendum on the project depends on the jury’s decision on the narrow vested-rights issue.

“We’re not aware of a similar instance in which the fate of a large energy asset rests in the hands of a jury. This is an unusual circumstance,” Timothy Fox, vice president of Clear View Partners, an energy research firm in Washington, D.C., said before the trial began in a packed courtroom Monday.

Attorneys for groups opposed to the project suggested to jurors on Monday that developers rushed construction with a goal of winning vested rights and nullifying the referendum.

But John Armando, lawyer for the developers, said the construction schedule was put in place years earlier, and that the case is “about fundamental fairness, about vested rights, about protection of property rights against retroactive laws.”

Last year, the Maine Supreme Judicial Court breathed new life into the stalled project when it ruled the retroactive nature of the statewide vote to stop the project would violate the developers’ constitutional rights if substantial construction already had begun in good faith before the referendum. Construction started in January 2021, about 10 months before the referendum in which 59% of voters rejected the project.

Justice Michael A. Duddy could have made the fact-finding determination himself but ruled in favor of project opponents  who asked for a jury to make the determination. That ruling is considered a victory for those who are opposed to the project.

Central Maine Power’s parent company and Hydro Quebec teamed up on New England Clean Energy Connect, which was unveiled in 2017 with a goal of supplying up to 1,200 megawatts of Canadian hydropower to the New England power grid. That is enough electricity for 1 million homes.

The project encountered opposition each step of the way even as it received all necessary regulatory approvals. Developers already had begun cutting trees and setting poles for months when the governor asked for work to be suspended after voters rejected the project in November 2021.

Supporters say bold projects such as this one, funded by ratepayers in Massachusetts, are necessary to battle climate change and introduce additional electricity into a region that is heavily reliant on natural gas, which can cause spikes in energy costs.