When John McCain is asked how he’s substantively different from George W. Bush, the senator has a programmed response he repeats without hesitation. At the top of the list: “climate change.”

Indeed, McCain, at least recently, has made his “green” credentials a key part of this early stage of his campaign. Two weeks ago, he traveled quite a bit to talk up his plan to combat global warming. “I’m proud of my record on the environment,” McCain told reporters. “As president, I will dedicate myself to addressing the issue of climate change globally.”

There are more than a few problems with McCain’s boasts. For one thing, McCain’s environmental positions shift all the time, and some of his policy opinions contradict each other. For another, McCain has a nasty habit of promoting environmental policies he’s already voted against.

Nevertheless, McCain wants voters to consider him someone who’ll work hard to address global warming if he’s in the White House. In the meantime, though, he’s not willing to work hard at all to address global warming while he’s in the Senate.

While Sen. John McCain (R-Ariz.) has made action on climate change a central theme in his campaign, he won’t be on hand to vote next week when the Senate considers a landmark bill imposing mandatory limits on greenhouse gases.

In a press conference late Wednesday afternoon, McCain said he did not support the bill sponsored by two of his closest allies, Sens. Joseph I. Lieberman (I-Conn.) and John Warner (R-Va.) because it doesn’t offer enough aid to the nuclear industry, and he would not come to the floor to vote on it.

“I have not been there for a number of votes. The same thing happened in the campaign of 2000,” he said. “The people of Arizona understand I’m running for president.”

That’s probably true. But the people of the country also understand that he’s promising to be a “leader” on one of the nation’s most pressing issues. And I’ll give McCain a tip: leaders show up for work once in a while.