But Republican leaders also maintained that their opposition was based on her behavior during a tree-spiking incident in 1989, not her opposition to an expansion of oil and gas drilling.
As a graduate student in Montana, Ms. Stone-Manning, now 56, retyped and mailed a letter to the U.S. Forest Service warning about a plan to insert metal spikes into trees in Clearwater National Forest in Idaho. At the time, “tree spiking” was a tactic used by some environmentalists to damage logging machinery. But it could also harm or even kill the workers using that equipment.
Ms. Stone-Manning testified in the Clearwater case, helping to convict two of the men involved, and had described her decision to type the letter as a way of trying to warn the authorities.
Republicans have accused Ms. Stone-Manning of lying about her connection to the incident and branded her an “eco-terrorist.”
“It’s hard to imagine a nominee more disqualified than Tracy Stone-Manning,” said Senator John Barrasso, Republican of Wyoming and ranking member of the Senate Committee on Energy and Natural Resources. “Every Senate Democrat will be responsible for her confirmation.”
Democrats stood unanimously behind Ms. Stone-Manning on Thursday. They noted that the tree-spiking incident occurred when she was in her 20s and argued that she was aiding the authorities. They described her as someone who spent the subsequent decades building bridges between environmentalists, ranchers and fossil fuel interests.
“She is someone who knows the value of collaboration, she is someone who can listen, who can reason, that knows our public lands, that’s recreated on our public lands her whole life,” said Senator Jon Tester, Democrat of Montana, who employed Ms. Stone-Manning as an aide and has known her for two decades.
Most recently Ms. Stone-Manning was the senior adviser for conservation policy at the National Wildlife Federation, a nonprofit conservation group. She has also worked as an aide to former Democratic Gov. Steve Bullock of Montana and has served as the head of Montana’s environment agency.
Emily Cochrane contributed reporting.