Sen. John McCain’s (R-Ariz.) hesitation to endorse climate change legislation on the Senate floor this week has Democrats charging the presumptive GOP nominee with paying lip service to the issue.

Republicans are paying close attention to how McCain maneuvers in the debate, given that he’s at odds with many in his party on the topic. For some, his support for cap-and-trade policies is another example of McCain undermining conservative principles.

His pause in endorsing the bill, authored by allies Sens. Joe Lieberman (I-Conn.) and John Warner (R-Va.), has opened him up to attacks from Democrats who say his talk about controlling greenhouse gases doesn’t amount to his election-year actions.

“Sen. McCain is going to have to make a decision whether he wants to be the forward-looking ‘reformer’ that he wants people to believe he is, or pay attention to all the Republican interest groups who are pushing things back,” said Sen. Charles Schumer (D-N.Y.). “I don’t know if it is a flip-flop, but certainly I think a lot of people are disappointed that Sen. McCain is not supporting this modest bill which is supported by Sen. Lieberman, Sen. Warner — neither of them [are] flaming radicals.”

The bill pending before the Senate would cap emissions of greenhouse gases 19 percent below current levels by 2020 and 71 percent by 2050. It is modeled after a landmark bill McCain and Lieberman forced a vote on in 2003, which would have capped emissions by 2010 at year 2000 levels.

After nearly every Republican voted against that bill, which failed 43-55, McCain and Lieberman reintroduced the bill in 2005 and later revised the measure at the beginning of the 110th Congress. The latest bill would gradually reduce emissions so that overall emissions would amount to one-third of 2000 levels by 2050.

That position differs sharply from President Bush, who has strongly resisted a mandatory cap on emissions.

As Democrats have tried to portray a McCain presidency as a third Bush term, the Republican has highlighted his advocacy of a cap on emissions as a clear break with the president. In a highly publicized speech last month in Portland, Ore., McCain said that the evidence of climate change was “compelling,” including the disappearance of glaciers and polar ice sheets, extreme weather events and the risk to wildlife.

“We have many advantages in the fight against global warming, but time is not one of them,” McCain said, saying the cap-and-trade system would replicate the “enormous success” of controlling acid rain in the 1990s.

“The difference between [McCain], President Bush and many members of the conference is well-known,” said Sen. Lindsey Graham (R-S.C.), a loyal McCain ally. “I think it’s one of the areas he can say, ‘If you want change in policy, this is change.’”

A spokesperson for McCain did not respond to requests for comment.

But as McCain has tried to create distance from Bush, he also risks alienating conservatives who have been wary about his positions on issues like campaign-finance reform and immigration.

Conservatives say that his advocacy of a cap-and-trade system undermines his claims that he is a conservative out of the mold of Ronald Reagan, who sought a smaller and more limited government.

“It’s inconsistent, I think, with those principles,” said Sen. Jeff Sessions (R-Ala.). “McCain is strong on a lot of key issues; he’s strong on judiciary; he’s strong on wasteful spending. But cap and trade is one that worries me a lot because it doesn’t work.”

“He certainly is a fiscal conservative, maybe he should qualify it that way,” said   Sen. James Inhofe (R-Okla.), who has famously dubbed manmade global warming a “hoax.”

Republicans spent Tuesday piling on the Lieberman-Warner bill, saying it would drive up gas prices already hovering around $4 a gallon. At a morning press conference, Minority Leader Mitch McConnell (R-Ky.) ducked questions about McCain’s support for the market-based pollution control system, but said, “I personally don’t like the cap-and-trade approach.”

Other members of his conference were more strident in their opposition. Sen. Kit Bond (R-Mo.) likened a government-run cap-and-trade system to a “Soviet model or the Chinese model. ... You could have a bunch of czars, a small group of czars sitting around saying, ‘We’re going to take money from this industry and give it to this industry,’ ” referring to a government-administered auction system for pollution credits.

“I think our position is truly conservative,” Bond said, referring to GOP plans to allow more access to energy supplies and invest in clean-energy technologies.

McCain has withheld his support because of concerns that it won’t promote nuclear energy, which does not produce emissions of carbon dioxide, a leading greenhouse gas.

But Lieberman thinks McCain eventually will come around to supporting the bill. He is trying to add language that would boost nuclear power by pumping money into the manufacturing of nuclear components and paving the way for workers and engineers at power plants.

“This is essentially the bill that he and I introduced five or six years ago,” Lieberman said. “I just think he’s going to support the bill — look, he was one of the first people who wanted to do something about global warming.”

But some Democrats see it as a reversal in positions.

“Well, I’m confused because he supported it before he opposed it,” said Sen. Barbara Boxer (D-Calif.) of McCain’s position.