Twice in the last three months, the US Senate has come within one vote of overturning $1.7bn in tax benefits for oil companies and using the money to promote renewable energy. Both times, McCain has skipped the vote, effectively killing the proposal and alarming leading green groups.
McCain also was a no-show during controversial votes on subsidising the conversion of oil to "clean" coal and relaxing rules for oil refineries.
When the League of Conservation Voters (LCV) released its annual environmental rankings last month, McCain - whose campaign website declares him "a leader on the issue of global warming" - earned a zero for missing all of the group's votes on key green issues. He was one of nine Republicans scoring the lowest possible rating.
"Every other member [of Congress] who received a zero from LCV last year at least had the temerity to show up and vote against the environment and clean energy time after time," Carl Pope, executive director of the Sierra Club, said in a reply to the scorecard.
"And unlike John McCain, I doubt any of them would claim to be environmental leaders or champions on global warming."
Other environmental groups have challenged the zero rating, defending McCain for missing the key votes while on the campaign trail. Republicans for Environmental Protection, which officially supports McCain's candidacy, has charged the LCV and the Sierra Club with undermining the green movement by criticising him.
Sierra has asked its estimated 1m members in the US to join the cause, however, writing letters to local newspapers publicising the zero rating and blasting McCain.
A brewing green backlash could deflate McCain's appeal to moderate voters in the US who identify with his rebellion from the party line. While McCain has distanced himself from past stances on immigration and taxes that hurt him with conservatives, his green credentials remain central to his maverick charm.
McCain introduced the Senate's first climate change bill in 2003, pioneering the concept of a "cap and trade" system for industries to swap carbon emissions credits. His LCV green score that year was 53%, with his overall rating since 2001 on 24%. Barack Obama has a lifetime rating of 86%, with Hillary Clinton earning a 90%.
Nick Berning, a spokesman for Friends of the Earth Action, said McCain's environmental record is strong only when compared with his fellow Republicans.
"When you compare him with the Democrats, he's light years away," Berning said. "McCain has sold himself as a real environmental champion. When you look at the record, that's just not the case."
The latest version of McCain's climate change bill would limit US greenhouse gas emissions to 60% of their 1990 levels by 2050. Obama and Clinton's climate proposals would cut emissions by 80%, the level that UK and US scientists have said would be needed to prevent dangerous temperature increases.
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This year's climate plan of choice, which is slated for a Senate vote this spring, uses the higher emissions levels of 2005 as a starting point and calls for a 70% cut by 2050. Two supporters of McCain's presidential bid drafted the legislation, but he has not endorsed it.
McCain's spokeswoman told the Washington Post last autumn that the senator has withheld support for the climate plan "because it doesn't include the nuclear issue by name".
David Jenkins, the government affairs director at Republicans for Environmental Protection, advised environmentalists to focus on the bigger picture and give McCain credit for endorsing the principle of mandatory emissions limits.
"People should be happy that both candidates [in the 2008 election] will have pledged to make fighting climate change a priority and will both be in favour of carbon caps," Jenkins said.
Penalising McCain for his opposition to broad bills that include a single environmental provision, Jenkins added, misrepresents his record on green issues. When the senator angered green activists by rejecting renewable energy support for the second time, for example, the environmental plan was attached to a broader economic aid bill that McCain opposed.
Dan Weiss, director of climate strategy at the liberal Centre for American Progress, agreed that McCain deserves credit for raising awareness of climate change in the face of resistance from George Bush.
But the self-described "straight talker" has rested on his laurels since, Weiss added: "He's been a former B student who's been cruising on his record, even though he's been a truant for the past year."
If McCain's Democratic opponent can take his environmental legacy down a peg, he stands to suffer with the swing voters who make up a crucial bloc in US presidential elections. Green issues are very important to independents unaffiliated with either US political party, according to a September poll by the group Environmental Defence.
That poll found 66% of independents preferring a candidate who wants the US to endorse an international pact limiting emissions regardless of whether China and India are included.
McCain has said he would support global pollution caps only if China and India also agree - a position backed by only 27% of swing voters and a minority of Republicans.
Despite McCain's zero rating, Berning advised the likely Republican nominee to consider a quick turnaround back to the forefront of the green movement.
"There's no question that the Democrats have staked out better positions than McCain. Once the primaries are done, it depends on where he positions himself," Berning said.
"He hasn't even put forward a detailed global warming plan yet."
© 2008 The Guardian