There was a time when Republicans were at the forefront of efforts to investigate – maybe even do something about – the impact of human activity on global climate.

John McCain was an early and persistent supporter of cap-and-trade efforts to reduce the greenhouse gases (mainly carbon dioxide) associated with climate change. So was Newt Gingrich, who went on to make a YouTube video ad – with then-House Speaker Nancy Pelosi, no less – where he said, “Our country must take action to address climate change.”

Now, Republican presidential hopefuls seem to be racing in the opposite direction – disavowing their past support for policy measures on climate – even any sense that there’s a problem to be addressed.

As Governor of Minnesota, Tim Pawlenty signed a state greenhouse gas law limiting emissions, led a regional climate partnership with Illinois, Iowa, Michigan, and Wisconsin, and he supported cap-and-trade. Since then, he’s flip-flopped.

At the Conservative Political Action Committee conference in Washington earlier this year he made his mea culpa.

“Have I changed my position? Yes," Pawlenty said. "But I'm not going to be cute about it, hem and haw, be dippy and dancy about it. Just saying yeah, it was a mistake. It was stupid. It was wrong."

Former US Rep. Sherwood Boehlert (R) of New York, a strong supporter of the environment, says he has "never been so disappointed in all my life in the pretenders to the throne from my party."

"Not one of them is being forthright in dealing with climate science," he told the Associated Press recently. "They are either trying to finesse it, or change previous positions to accommodate the far right. They are denying something that is as plain as the nose on your face."

To liberal critics, the answer is obvious: The influence of climate change denialists financially supported by the billionaire Koch brothers (David and Charles) and others tied to the oil industry.

Whether or not that’s true, there’s no denying that climate change (and environmental issues generally) carry less political weight than they did in recent years.

While a majority (52 percent, according to Gallup) still believes climate change is caused by human activities as opposed to natural causes (43 percent), that’s down from a high of 61 percent in 2007. Similarly, the number of Americans who believe the impacts of global warming already are being felt has dropped to fewer than half.

“Americans are clearly less concerned about global warming and its effects than they were a few years ago,” Gallup’s Jeffrey M. Jones reported in March. “The reasons for the decline in concern are not obvious, though the economic downturn could be a factor…. environmental concerns tend to take a back seat to economic matters when the economy is in poor shape.”

It’s also a clearly partisan issue. Much higher percentages of Republicans (i.e., conservatives) now say the issue has been overblown in the media (67 percent), and they’re much less likely to connect increases in temperature to human activities. Just 31 percent worry about it at all, less than half the rate for Democrats.

Thus, the shift in overall public attitude is largely driven by the growing influence of politically conservative thought, especially the tea party. That, in turn, drives the actions and assertions of presidential hopefuls facing a gantlet of party conservatives in next year’s primary elections and caucuses.

Presumed front-runner Mitt Romney’s tack has been interesting.

He has not given it the full genuflect that Pawlenty and Gingrich did.

(Campaigning in Manchester, New Hampshire, in late May, Gingrich, the former cap-and-trader, suggested that climate change is "the newest excuse to take control of lives" by "left-wing intellectuals." Campaigning at a house party, Gingrich was asked about “the climate/global warming problem.” He went off on a long professorial toot in which he pointed out that as an “amateur paleontologist” he observed that “It was substantially warmer in the age of dinosaurs when there were no cars, at least not that we’ve been able to find, and no factories.”)

Romney was more direct.

"I believe the world is getting warmer, and I believe that humans have contributed to that," he told a crowd at a town hall meeting in Manchester. "It's important for us to reduce our emissions of pollutants and greenhouse gases that may be significant contributors."

It was an “aha!” moment for ideological purists. “Bye, bye nomination” declared Rush Limbaugh, who says global warming is “one of the most preposterous hoaxes in the history of the planet.”

As the headline on one Huffington Post piece put it: “Mitt Romney Attacked For Being Reasonable About Climate Change”

Still, it’s not that Romney’s position differs at all from majority Republicans in the House, where cap-and-trade is dead and there’s unlikely to be any legislation addressing climate change.

Romney did get a shout-out from Al Gore.

“Good for Mitt Romney," Gore blogged. "While other Republicans are running from the truth, he is sticking to his guns in the face of the anti-science wing of the Republican Party."

Which was a little like President Obama praising Romney for the Massachusetts model on health care.