WASHINGTON (Reuters) - Billionaire environmental activist Tom Steyer will give a boost to 2014 political candidates from seven U.S. states who work to combat climate change, countering political support from fossil fuel interests.

NextGen Climate, Steyer's political group, said Thursday it would back candidates in Colorado, Florida, Iowa, Michigan, Maine, New Hampshire and Pennsylvania who face challenges from opponents who either doubt that humans cause climate change or receive donations from the fossil fuel industry.

NextGen said it would use climate change as a "wedge issue" to drum up voter turnout and to show that taking an anti-science position can hurt rather than help political candidates.

"The debate on climate change is settled," Steyer said. "It is here, it is human-caused, and it is already having a devastating impact on our communities, but we need to accelerate the level of political support to address this critical issue before it's too late."

Steyer said earlier this year that he planned to put $50 million of his own funds toward backing candidates in the November elections who support action on climate change. He said he also hoped to mobilize like-minded donors to match that amount.

A self-styled counterweight to libertarian megadonors Charles and David Koch, Steyer has quickly become a major force in U.S. politics.

His financial clout and focus on environmental issues have given Democratic candidates a reason to highlight their stance on climate protection rather than hide their views, as has happened in the past.

In Colorado, a state where energy and environment issues are front and center, NextGen said it would oppose Republican Senate candidate Cory Gardner, who has denied that human beings contribute to climate change and opposes Environmental Protection Agency rules to combat carbon pollution.

NextGen is supporting Democratic incumbent Senator Mark Udall even though he has not stated his position on the Keystone XL pipeline, a key concern for Steyer.

In the Florida governors' race, NextGen said it was going after Governor Rick Scott, who has publicly doubted humans' contribution to climate change despite his state's vulnerability to the rising sea levels it causes.

Steyer stepped down in 2012 as co-managing partner of Farallon Capital Management, the hedge fund he founded in 1986, to devote himself to full-time activism.

(Reporting by Valerie Volcovici; Editing by Lisa Von Ahn)