"The world is already feeling the powerful effects of global warming," Republican presidential candidate Sen. John McCain of Arizona said this week in a policy speech. "The problem isn't a Hollywood invention nor is doing something about it a vanity of Cassandra-like hysterics. It is a serious and urgent economic, environmental and national security challenge."

McCain is hardly alone in his party. Rudolph Giuliani and Mitt Romney have also staked out positions on this topic, generally pushing for alternative energy and more efficient technologies to stall the globe's warming trend.

One reason for this sea change is former Vice President Al Gore's movie "An Inconvenient Truth" laying out the science behind global warming, said Eileen Claussen of the non-profit Pew Center on Global Climate Change.

This new Republican openness to discussing ways to tackle climate change may also be an indirect result of recent changes in how the White House has talked about the problem, said political scientist Cal Jillson of Southern Methodist University in Dallas said.

"Once President George W. Bush acknowledged that global warming was a problem, other Republicans were freer not to have to support the administration stance -- that the science was incomplete -- but now they could say on the campaign trail, 'Here's what I propose to do.'"

This shift has occurred in the last year or so, Jillson said.

CHANGING ATTITUDES

Global warming surfaced on Thursday at a debate among eight Democratic presidential hopefuls in South Carolina, where more than two-thirds of the members of the state's House of Representatives sent an open letter to the White House candidates asking them to make energy and climate change a campaign priority.

It wasn't the top issue -- the war in Iraq and health care came first -- but it elicited firm responses from Illinois Sen. Obama, who said he organized a tree-planting and switched to energy-efficient lightbulbs in his home, and Delaware Sen. Joseph Biden, who called for a major effort to deal with global warming and energy issues.

The fact that the question was asked at all is in line with increasing concern on this problem, as illustrated by a New York Times/CBS News poll released on Friday.

The poll found most Americans felt it was more important to protect the environment than stimulate the economy, most favored requiring car makers to make more efficient vehicles and said they would be willing to pay more for electricity from renewable sources if it reduced global warming.

Claussen said the issue was avoidable in the last two presidential elections, but won't be in the 2008 race.

"It is a must-have for every presidential candidate," Claussen said.

Both Claussen and Jillson said the increasing certainty of the environmental science has pushed the issue to the front of political consciousness.

"They will all have to talk about it," Jillson said. "The Democrats will talk about it as indisputable, that global warming is caused by human activity and needs to be addressed with law and regulation.