The Wilderness Society, the Sierra Club and other conservation groups fighting the 303-mile pipeline urged the 4th U.S. Circuit Court of Appeals on Monday, in two separate cases, to declare the mandate unconstitutional and keep their challenges alive.

The environmental groups' lawsuits are seeking to invalidate key federal permits that are needed to finish construction of the pipeline, including on a stretch of land running through a federal forest in Virginia.

They said the separation of powers doctrine allows Congress to write the law, but not to directly determine the outcome of court cases.

"Congress cannot pick winners and losers in pending litigation by compelling findings or results," the Wilderness Society wrote in its filing.

Pipeline developer Equitrans Midstream Corp. said Tuesday it doesn't comment on pending litigation.

The U.S. Forest Service declined to comment Tuesday and the U.S. Fish and Wildlife Service didn't respond to a request for comment. Those agencies issued the challenged federal approvals for the project.

The U.S. government had asked the court to dismiss the cases on June 14 arguing that the bill stripped the court of jurisdiction to hear the lawsuits.

The developers on Monday announced the project has received all necessary federal permits, and asked the U.S. Federal Energy Regulatory Commission for final approval to restart construction.

If completed, the pipeline would transport natural gas across West Virginia and Virginia. Mountain Valley Pipeline LLC - a joint project between Equitrans, NextEra Energy Inc., Consolidated Edison Inc., AltaGas Ltd and RGC Resources Inc. - has said the pipeline is over 94% complete.

The 4th Circuit, where most of the litigation challenging the project has occurred, has previously vacated several of the project's federal and state permits - some more than once - over concerns about the pipeline's environmental impacts.

The current lawsuits target the Forest Service's approval of a right-of-way that would allow the pipeline to pass through the Jefferson National Forest, and a Fish and Wildlife Service approval that determined the construction wouldn't likely jeopardize endangered animals.

The challengers have claimed the approvals inadequately considered the environmental impacts of the pipeline to wildlife and the lands it would cross.

Congress included a rider in this month's debt ceiling bill mandating that federal agencies issue approvals for the pipeline, and restricting judicial review of would-be and existing challenges.

The cases are Appalachian Voices et al. v. United States Department of the Interior and the Wilderness Society v. U.S. Forest Service, in the 4th U.S. Circuit Court of Appeals, case Nos. 23-1384 and 23-1592.

For the Sierra Club and Appalachian Voices group: Elizabeth Benson of the Sierra Club and Derek Teaney of the Appalachian Mountain Advocates.

For the Wilderness Society: Gregory Buppert and Spencer Gall of the Southern Environmental Law Center.

For the U.S. government: Allen Brabender and Kevin McArdle of the U.S. Department of Justice.

For the developer: Jeffrey Lamberson and George Sibley of Hunton Andrews Kurth.

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(Reporting by Clark Mindock)

By Clark Mindock