WASHINGTON — In December 2005, Republicans were poised to open the Arctic National Wildlife Refuge up for oil and gas drilling, an achievement they had sought for decades. The chairman of the Senate Appropriations Committee, Ted Stevens, a Republican of Alaska, had attached the provision to a must-pass defense spending bill and threatened to keep lawmakers in Washington until Christmas if they tried to strip it. Desperate to remove the provision, leaders from national environmental groups turned to a handful of key Republican senators for help.

With only days left before the critical vote, the president of the League of Conservation Voters, Gene Karpinski, and the president of the Defenders of Wildlife Action Fund, Rodger Schlickheisen, obtained a private audience with Senator McCain. Mr. McCain had been on both sides of the Arctic drilling issue over the course of his career, and the two leaders of the fight against opening the refuge were eager to know whether he would come down in their column.

His answer disappointed them. In the brief meeting, the senator said he was unwilling to risk blocking a bill involving the military at a time of war — even though it was clear the broader funding bill would pass quickly and by a wide margin if opponents managed to strip the ANWR provision from it. “We told him, ‘This may be the key vote, this may be the time we win this,'” Mr.Schlickheisen recalled in an interview. “He said, ‘Not on this bill.’ That was it.”

Ultimately environmental activists were able to defeat the measure with the aid of two Republican senators — Lincoln Chafee of Rhode Island and Mike DeWine of Ohio. But they have not forgotten Mr. McCain’s decision, and many say it exemplifies his approach to environmental issues.

“There’s no question that among a lot of bad Republican votes in the Senate, he’s one of the better ones,” Mr. Schlickheisen said. “He is perhaps the most unpredictable, erratic, of those votes.”

Mr. McCain has made the environment one of the key elements of his presidential bid. He speaks passionately about the issue of climate change on the campaign trail, and he plans to outline his vision for combating global warming in a major speech today in Portland, Ore.

“I’m proud of my record on the environment,” he said at a news conference Friday at the Liberty Science Center in Jersey City. “As president, I will dedicate myself to addressing the issue of climate change globally.”

But an examination of Mr. McCain’s voting record shows an inconsistent approach to the environment: He champions some “green” causes while casting sometimes contradictory votes on others.

He has been resolute in his quest to impose a federal limit on greenhouse gas emissions, even when it means challenging his own party. But he has also cast votes against tightening fuel-efficiency standards and resisted requiring public utilities to offer a specific amount of electricity from renewable sources. He has worked to protect public lands in his home state, winning a 2001 award from the National Parks Conservation Association for helping give the National Park Service some say over air tours around the Grand Canyon, work that prompts interior secretary and Governor Babbitt of Arizona to call him “a great friend of the canyon.” But he has also pushed to set aside Endangered Species Act protections when they conflict with other priorities, such as the construction of a University of Arizona observatory on Mount Graham.

Mr. McCain’s senior policy adviser, Doug Holtz-Eakin, said the senator does not always please “environmental groups who are single-issue, litmus test” organizations. Instead, he said, Mr. McCain seeks to weigh the costs and benefits of each environmental issue.

“Look, he always balances what are the environmental implications of these enterprises and what are the economic benefits that could come from them,” Mr. Holtz-Eakin said. “That is, in general, an approach which may be harder to read than a flat ideological X or Y, but it’s how he reads these things, it’s how he evaluates these kinds of decisions.”

As a result, Mr. McCain scores significantly lower than his Democratic rivals for the presidency, senators Obama and Clinton, in interest groups’ studies of his environmental voting record. Mr. McCain’s lifetime League of Conservation Voters score is 24%, compared with 86% for Mr. Obama and 86% for Mrs. Clinton; Defenders of Wildlife Action Fund’s conservation report card gave him 38% in the 108th Congress and 40% in the 109th. (Mr. McCain has missed every major environmental vote this Congress, giving him a zero rating.)

When Mr. Karpinski tells audiences about McCain’s environmental scorecard rating, he said, “jaws drop. … I tell them, ‘He’s not as green as you think he is.'”