HOW MAVERICKY…. Even during the 2008 presidential campaign, no matter how far he shifted to the right, John McCain was generally pretty good about acknowledging the climate crisis. “We need a successor to Kyoto, a cap-and-trade system that delivers the necessary environmental impact in an economically responsible manner,” McCain said in 2008.

Now, the specifics of McCain’s cap-and-trade proposal were pretty absurd — it was basically cap and trade without the cap — but at least he’d occasionally talk a good game. Now, the Arizona Republican is poised to help kill a compromise measure that the nation really needs.

Sen. John McCain now appears to oppose climate-change legislation, an abrupt switch that could seriously threaten any movement on such a bill.

“Nearly 1000 page Climate Change legislation — appears to be a cap & tax bill that I won’t support,” McCain wrote in a Twitter message Monday, a reversal of the position he took on the Senate floor in March.

Two months ago, McCain and his close friend Sen. Lindsey Graham, the South Carolina Republican, took the floor in strong support of climate-change legislation. This marked a return to form for McCain, who co-sponsored a 2002 climate-change bill with longtime friend Sen. Joseph Lieberman (I-Conn.), but had tamped down his rhetoric during the 2008 presidential campaign.

“Let me just say to my colleagues, I’m proud of my record on climate change,” McCain said in March. “I’ve been all over the world and I’ve seen climate change, and I know it’s real, and I’ll be glad to continue this debate with my colleagues and people who don’t agree with that.”

Keep in mind, while reconciliation rules are in place for health care, center-right Democrats made it so that Republican obstructionism can kill climate-change legislation. To get to 60, Dems are going to need quite a few votes from those handful of Republicans who take science and global warming seriously.

McCain was supposed to be one of them. That now appears unlikely.

McCain had an opportunity to help bolster his tarnished reputation, regaining the stature he enjoyed after the 2000 campaign. Instead, he’s throwing the opportunity away. He’s gone through a variety of personas over the years, but it now seems he’s sticking with the one rejected by the country in 2008.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.