A dump truck descends a steep hill along the NECEC corridor in Johnson Mountain Township in 2021. A federal judge recently decided not to suspend permits for the 145-mile transmission corridor. Gregory Rec/Portland Press Herald, file

A federal judge has decided not to revoke permits for the controversial New England Clean Energy Connect corridor, effectively ending a long-running legal battle — but not until more than a third of the project had already been completed.

Chief U.S. District Judge Lance Walker on Monday ruled in favor of the U.S. Army Corps of Engineers and the U.S. Department of Energy, after several environmental groups sued the federal agencies over their environmental analysis of the $1.5 billion 145-mile transmission corridor to bring Canadian hydropower to the New England grid.

The Appalachian Mountain Club, Natural Resources Council of Maine and Sierra Club Maine in 2020 alleged the Army Corps failed to rigorously assess the transmission corridor project’s environmental impact on 53 miles of undeveloped forestland in western Maine.

The Army Corps previously determined the project would have “no significant impact” on the environment, a determination that the environmental groups argued was reached without adequate due diligence.

They said the agencies “cherry-picked” environmental impacts instead of considering them holistically when evaluating alternatives and took a quality-over-quantity approach to its analysis, focusing on the acreage impacted instead of the impact on the aquatic ecosystem.

But in a 59-page decision, Walker disagreed and found the Corps analysis was thorough and appropriate.

Walker said the judgment was not a “comprehensive statement” about the transmission project as a whole.

The environmental groups also took umbrage with the Department of Energy’s decision not to circulate a draft environmental document for comment after saying it would.

Walker agreed that reneging on a promise was “discourteous” but said it was not arbitrary and capricious, as alleged.

It was not immediately clear if the environmental groups plan to appeal.

Maine voters rejected the corridor in a 2021 referendum, but developers were allowed to resume construction after a Cumberland County jury found, in a separate suit, that the project’s developer had established “vested rights” to continue building.

The corridor is expected to be finished by the end of the year. The project is organized under NECEC Transmission LLC, a subsidiary of Connecticut-based Avangrid Inc., the parent company of Central Maine Power Co. Both are controlled by Iberdrola, a Spanish multinational energy company.

The transmission line will connect 1,200 megawatts to the New England electrical grid.

Advocates say the project will help reduce greenhouse gas emissions. Opponents are skeptical of the environmental benefits and have said developers are gouging a path for the transmission line through some of Maine’s most pristine forested areas.