WASHINGTON - One thing that Tim Pawlenty, Jon Huntsman, Newt Gingrich and Mitt Romney have in common: These GOP presidential contenders all are running away from their past positions on global warming, driven by their party's loud doubters who question the science and disdain government solutions.

All four have stepped back from previous stances on the issue, either apologizing outright or softening what they said earlier. And those who haven't fully recanted are under pressure to do so.

The latest sign of that pressure came Thursday when New Jersey Gov. Chris Christie said he was pulling his state out of a regional agreement to reduce greenhouse gases, saying it won't work. While Christie, a rising GOP star, has said he's not running for president, some in the party continue to recruit him.

It's an indicator of a shift on the issue among conservative Republicans, who have an outsized influence in the party's presidential primary elections. Over the last few years, Gallup polling has shown a decline in the share of Americans saying that global warming's effects have already begun - from a high of 61 percent in 2008 to 49 percent in March. The change is driven almost entirely by conservatives.

In 2008, 50 percent of conservatives said they believed global warming already is having effects; that figure dropped to 30 percent this year. By contrast, among liberals and moderates there's been relatively little movement, and broad majorities say warming is having an impact now.

Not all Republicans are happy with the trajectory the party is on when it comes to global warming. Former New York Rep. Sherwood Boehlert, a 27-year veteran of Congress who was known as a staunch protector of the environment, said he has "never been so disappointed 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 said in an interview. "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."

But for some, opposing mandated solutions to climate change has become party orthodoxy.

"Republican presidential hopefuls can believe in man-made global warming as long as they never talk about it, and oppose all the so-called solutions," said Marc Morano, a former aide to Oklahoma Sen. James Inhofe, one of the most vocal climate skeptics in Congress.

Morano now runs a website called Climate Depot where he attacks anyone who buys into the scientific consensus on climate change. Enemy No. 1 for Morano these days is Gingrich, the former House speaker who in 2008 shared a couch with then-House Speaker Nancy Pelosi in a TV ad backed by climate change guru Al Gore.

In it Gingrich says, "We do agree that our country must take action on climate change."

Since that appearance, Gingrich, who once ran an environmental studies program at a Georgia college, has called for the abolition of the Environmental Protection Agency. He's also spoken out against a Democratic bill that passed the House in 2009 that would have limited emissions of greenhouse gases and created a market for pollution permits to be bought and sold.

But that hasn't been enough to satisfy conservative critics. Gingrich went further in a recent interview when he said he doubted there was a connection between climate change and the burning of fossil fuels.

"The planet used to be dramatically warmer when we had dinosaurs and no people," Gingrich told The Macon (Ga.) Telegraph last week. "To the best of my knowledge the dinosaurs weren't driving cars."

Where Gingrich has waffled, other GOP contenders have conceded on the issue of climate. Romney, Pawlenty and Huntsman potentially come into the race with even more climate baggage, since each of them as governor supported a regional "cap-and-trade" program to reduce greenhouse gas pollution, like the one Christie bailed out of this week. All have since abandoned that stance.

It's a marked turnaround for a party that just three years ago gave its presidential nomination to Sen. John McCain, who long has supported cap and trade to reduce greenhouse gas pollution and who campaigned on the issue even though it put him on the same side as his opponent, Barack Obama.

In fact, the whole idea of a market to trade pollution credits came from the Republican Party. It emerged in the late 1980s under the administration of President George H.W. Bush as a free-market solution to the power plant pollution that was causing acid rain. It passed Congress nearly unanimously in 1990 as a way to control emissions of sulfur dioxide.

But now it has become synonymous with partisanship and political risk. Legislation to use the pollution credits approach to curb global warming passed the Democratic-controlled House in 2009, with the support of Obama. It died in the Senate after Republicans labeled it a "cap-and-tax" plan that would raise energy prices and after House Democrats who voted for it were attacked at town hall meetings back home.

Many of those Democrats lost their seats in last November's elections and with the House now under Republican control, Obama has said he no longer would pursue it.