It wasn’t long ago that Marco Rubio and Tim Pawlenty -- two rising Republican stars -- supported legislation to limit greenhouse gas emissions. But in recent weeks, both have begun to express doubts about whether cars, factories and power plants have anything to do with global warming.

The shift by Rubio and Pawlenty -- as well as other prominent Republicans -- reflects the rising power of climate change skeptics in the GOP, where global warming is becoming a litmus test for conservatives.

Rubio, former speaker of the Florida House of Representatives, is running for the U.S. Senate. Pawlenty, Minnesota’s governor, is eyeing a 2012 presidential bid.

For Republicans, “the new political expediency is to be a global warming skeptic,” said Marc Morano, executive editor of the skeptic clearinghouse website ClimateDepot.com and a former aide to outspoken skeptic Sen. James M. Inhofe (R-Okla.).

Fuel for the resurgence of attacks on global warming came in December, when leaked e-mails from a British university showed top climate scientists from around the world apparently discussing skirting public information laws and other practices of questionable ethics -- an incident that has become known as Climategate.

Then came revelations of flaws in a key United Nations report, including a claim -- not supported by scientific evidence -- that Himalayan glaciers could disappear because of warming by 2035.

“It’s given opponents of climate action leverage to attack what was widely considered to be unimpeachable, which is the science,” said John R. Russell IV, a former Republican congressional staffer who now lobbies for renewable energy interests with Sonnenschein Nath & Rosenthal.

Fearing that the public is losing confidence in the science, researchers are fighting back by engaging their critics more directly, acknowledging errors and perception problems and encouraging the release of more raw data.

And while the issue has made for fiery campaign rhetoric, it hasn’t appeared to sway the close to a dozen Republicans in the Senate who still appear open to emissions restrictions, largely because of the prospect of new jobs and reduced dependence on foreign oil that could come from the development of alternative energy sources.

But polls suggest the scandals have shaken some voters’ belief in climate change, particularly conservative activists who widely regard emissions limits as a United Nations-fueled intrusion on personal freedom.

A recent online survey by climate change researchers at Yale and George Mason universities showed the percentage of Americans who are “dismissive” of climate science has more than doubled since 2008, from 7% to 16%.

A core group of dissenters has long challenged the prevailing scientific belief that fossil fuel combustion and deforestation are warming Earth and probably setting the stage for rising seas, increased drought and other dangerous effects of a changing climate.

But the academic arguments began to transform into political action after former Vice President Al Gore and the U.N. Intergovernmental Panel on Climate Change won the 2007 Nobel Peace Prize for their efforts to raise public awareness of global warming.

The skeptics had taken a body blow in the scientific debate, but that seemed to galvanize a grass-roots political effort to fight against the ensuing threat of emissions legislation.

President Obama mounted an aggressive push for a climate bill that passed the House in June, feeding a growing conservative backlash against initiatives, such as the healthcare overhaul and automaker bailouts, that activists called impingements on personal freedom.

If Climategate hadn’t broken when Obama was pushing emissions limits and world leaders were trying to reach a climate accord in Copenhagen last year, “it would not have had the impact that it had,” said Christopher Horner, a longtime climate science dissenter with the free-market Competitive Enterprise Institute in Washington, which has taken money from oil companies. “At this time and place, it resonated.”

That’s particularly true among Republican candidates.

Rubio has pummeled his primary opponent, Florida Gov. Charlie Crist, over Crist’s support for emissions limits. Pawlenty, who once joined with Democrats in backing emissions limits in a radio campaign, recently said on NBC’s “Meet the Press” that there are questions of how much climate change is man-made.

Even 2008 GOP presidential candidate John McCain, who argued often with climate skeptics on the primary campaign trail, recently played along with a Fox News interviewer who mocked global warming. In the face of a stiff primary challenge for his Senate reelection, McCain has backed away from his support for emissions limits.

One leading Republican, Sen. Lindsey Graham of South Carolina, warned his party last week that embracing skeptics could pose long-term problems for the party by alienating young voters concerned about climate change. Graham has been trying to hammer out a compromise climate and energy bill with Sens. John F. Kerry (D-Mass.) and Joe Lieberman (I-Conn.).

White House officials and environmentalists lobbying for a bipartisan climate bill say the science scandals have not cost them any potential swing votes, Republican or Democrat.

Many of the same polls that show some Americans doubting climate science also show support for emissions limits to boost energy independence and create “clean energy” jobs -- the arguments supporters have used to sell the climate bill for more than a year.

“The vast majority of people support the bill because of the benefits,” said Tony Kreindler, a spokesman for the Environmental Defense Fund, “and not because of the science.”

jtankersley@latimes.com