DALLAS/KANSAS - Sharon Byers is unconvinced that human activities such as the burning of coal and other fossil fuels are behind climate change.

There have been times in the past when there was global warming in the absence of man. It is all part of a natural cycle. I think it is a little vain to think man could destroy this great planet, said Byers, a former nurse who lives in Lee's Summit, Missouri.

In the U.S. heartland, global warming talk is often seen as hot air and opinion polls show skepticism on the rise, fueling conservative opposition to a climate change bill that is a priority for President Barack Obama and making some Democrats vulnerable in the November 2010 congressional elections.

America will pledge at U.N. climate talks in Copenhagen that begin next week to cut its greenhouse gas emissions, which most scientists say are the main agents of climate change, by 17 percent below 2005 levels by 2020.

In global scientific circles, that is seen as vital as America accounts for around a fifth of global emissions. But for some conservative U.S. activists it all seems sinister.

We're not interested in one-world government, which seems to be the direction of this summit, said Dale Robertson, the founder of the Tea Party organization which has held rallies across the country to protest Obama's agenda.

A constant theme on conservative and Christian talk-radio stations, which reach tens of millions of Americans, is the notion that the global warming scare is a hoax aimed at crippling the U.S. economy and way of life.

This all strikes a chord in these tough economic times and opinion polls show Americans cooling to the issue.

A Washington Post-ABC News poll in November found that 72 percent of Americans surveyed believed global warming was happening, down from 80 percent last year.

But even among Americans who accept that temperatures are climbing there is reluctance to pin the blame on humans.

A Pew poll in October found 57 percent of Americans felt there was evidence that the earth was warming, down from 71 percent in April 2008. But only 36 percent attributed this to human activities, compared with 47 percent last year.

TOUGH SELL, ELECTORAL RISKS

This makes the task of selling legislation to the U.S. public to cap emissions more difficult.

Getting legislation to this effect next year in the U.S. Senate is high on Obama's agenda. The House of Representatives has narrowly passed its own version and Republicans see opportunities here in next year's congressional contests when the Democrats will be fighting to maintain their majorities.

In the House of Representatives, Republican strategists have said that Harry Teague of New Mexico and Betsy Markey of Colorado are among the many Democrats seen vulnerable on this issue in 2010.

In the 100-seat Senate, some Democrats who are up for reelection next year will be in a bind on the issue, much as some are now with the healthcare debate.

If the Senate does vote on climate change at some point next year, certainly Sens. Barbara Boxer (California), Michael Bennet in Colorado, Blanche Lincoln in Arkansas, Arlen Specter in Pennsylvania and Harry Reid in Nevada could all be vulnerable, depending, of course, on how they vote, said Jennifer Duffy of the nonpartisan Cook Political Report.

Senator Joseph Lieberman, an independent, said on Thursday negotiators in the Senate are nowhere close to writing details of a compromise climate change bill and that at least two key Senate committees, Finance and Agriculture, have not yet worked on their portions of a bill.

Until then, a compromise bill will not be drafted, he said.

Analysts say growing public skepticism on the issue is explained in part by the recession and job losses.

Up until a couple of years ago when people felt relatively wealthy and secure, they were willing to consider climate change as a problem we should address. But now that they feel more poor and vulnerable they are skeptical, said Cal Jillson, a professor at Southern Methodist University in Dallas.

There are other uniquely American cultural traits that drive views on the issue, such as an enduring love affair with big vehicles which has been dented though hardly crushed by last year's record-high gas prices.

One in four U.S. adults is also an evangelical Christian and, while secular Europeans may find this odd, many really do believe that biblical prophecy foretells the planet's end.

If you are an evangelical Christian in the American vein then you believe it is our responsibility to look after the planet but it will be ultimately destroyed no matter what we do, said Bart Barber, a Southern Baptist Convention preacher in the small north Texas town of Farmersville.

(Additional reporting by Tim Gaynor in Phoenix and Richard Cowan in Washington; Editing by Paul Simao)