(Adds Reid quote from start of marathon session, paragraph 2)
By Thomas Ferraro and Valerie Volcovici
WASHINGTON, March 10 (Reuters) - More than two dozen U.S. Senate Democrats joined forces to speak through the night on Monday, hoping to "wake up" Congress to what is seen as the threat of climate change.
"Despite overwhelming scientific evidence and overwhelming public opinion, climate change deniers still exist," Senate Majority Leader Harry Reid said. "They exist in this country and in this Congress."
The Nevada Democrat made the comments in kicking off the marathon gabfest at 6:27 p.m. (2227 GMT) on Monday. The final address is expected to end about 15 hours later, at 9 a.m. (1300 GMT) on Tuesday.
Thirty of the Senate's 53 Democrats, plus the two independents who caucus with them, have signed up to participate. The White House said it would post live tweets during the overnight session under the hashtag #up4climate.
"It's time for Congress to wake up and get serious about addressing this issue," said Senator Sheldon Whitehouse of Rhode Island, a leader of the Climate Action Task Force that organized the event.
Whitehouse told reporters that the talkathon was the "opening salvo" to a more coordinated, years-long push for comprehensive climate legislation.
"We hope that by staying up all night ... we will signal a new dawn of climate change action in Congress," added Senator Ed Markey of Massachusetts.
Many scientists attribute rising temperatures and some extreme weather events worldwide to the increased release of carbon dioxide from burning fossil fuels.
They fear that as carbon dioxide accumulates in the atmosphere, there will be more severe weather, including floods, hurricanes, tornadoes and drought.
After failing to make climate change a priority in his first term, and with no major climate legislation expected from a deeply divided Congress, Democratic President Barack Obama is addressing the issue by wielding his executive powers.
Democrats hope to make it an issue that will rally their liberal base for November's congressional elections.
VULNERABLE DEMOCRATS LIKELY TO BE NO-SHOWS
Not signed on for Monday's event are four Democratic senators regarded as the most vulnerable in November: Mary Landrieu of Louisiana, Kay Hagan of North Carolina, Mark Begich of Alaska, and Mark Pryor of Arkansas.
Early indications were that none of the 45 Republicans in the Senate would participate either.
Republican lawmakers have been among the leading opponents of efforts to address climate change.
Shortly before Senate Democrats began their offensive, Republican Senator Jeff Sessions of Alabama rose to say that many were misinformed about the threat of global warming.
"There has been a lot of exaggeration, there has been a lot of hype," Sessions said. "It's time for us to be a bit more cautious and to be less alarmist and to focus more on the science of the situation."
Last week, Senate Republican leader Mitch McConnell of Kentucky told the Cincinnati Enquirer: "For everybody who thinks it's warming, I can find somebody who thinks it isn't."
McConnell has defended Kentucky's coal industry from moves by the Obama administration to limit emissions from power plants, which are blamed for contributing to global warming.
His comments drew a letter on Friday from Whitehouse and Democratic Representative Henry Waxman of California.
"Your statements are inconsistent with the overwhelming scientific consensus that climate change is occurring, is caused primarily by humans, and will have serious impacts if unchecked," the Democrats wrote.
Obama adviser John Podesta, a longtime advocate of climate action, joined the social media site Twitter on Monday before the all-nighter.
In just his second message, Podesta tweeted: "Climate deniers have closed their eyes (and minds) to what's happening to our planet. Stay #Up4Climate all night tonight with @SenateDems." (Reporting by Thomas Ferraro and Valerie Volcovici; Editing by Ros Krasny, Jonathan Oatis and Peter Cooney)
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