Voters in several key swing states have already started mailing in their November midterm ballots, and according to a poll conducted for three major environmental groups, they're becoming more attuned to a range of environmental issues, including climate change.

Hart Research contacted 1505 likely voters by mobile phones and landlines in Colorado, Iowa, Michigan, New Hampshire, and North Carolina and found that 54 percent of voters were more likely to favor candidates who advocate for action on climate change.

"This year is really crystalizing just how much of a liability climate science denial has been," national press secretary at the League of Conservation Voters Jeff Gohringer told VICE News. "You see candidates across the country — whether it's Cory Gardner in Colorado or Scott Brown in New Hampshire — trying to muddy the waters of where they stand on climate change because they know climate science denial is bad politics that puts them wildly out of step with voters."

Other green issues polled well. Sixty-six percent of swing state voters favored candidates who supported regulations to limit carbon dioxide emissions from power plants. And 68 percent of them had favorable views of candidates who backed clean energy programs, like solar and wind.

Only 23 percent of voters said they had a less favorable view of a candidate if they support action on climate change and curbing carbon dioxide pollution.

The poll was conducted for the League of Conservations Voters, NextGen Climate Action, and the National Resource Defense Council Action Fund. The margin of error for the poll was plus or minus 2.5 percentage points across all states.

LCV has invested $30 million in federal and state races across the country this election cycle and knocked on more than a million doors in Alaska, Colorado, Maine, Michigan and North Carolina. Combined, environmental groups are spending $85 million this election cycle.

That money, they say, is paying off, elevating environmental issues among the electorate and forcing formerly climate change denying politicians to change their tact.

"I don't think it's possible for a climate change denier to hold the White House," Heather Taylor-Miesle, the director of the NRDC Action Fund, told VICE News. "I think that's why you're seeing all this 'I'm not a scientist' jargon, rhetoric by Republicans."

Despite environmental issues strongly influencing voter attitudes, the Hart Research poll found that the topic ranked sixth among issues voters heard candidates talking about on the campaign trail. Obamacare and the combined topic of jobs and the economy topped the list.

"Climate change or environmental issues generally rank pretty close to the bottom," Karlyn Bowman of the American Enterprise Institute told VICE News. "That isn't because we don't care about the environment, it just doesn't have the same intensity among voters as other issues."

But this election cycle may be a turning point and with changing voter demographics environmental issues may get more attention.

The Yale Project for Climate Communication (YPCC) also released a poll on Thursday that found millennials, unmarried women, and people of color — what it calls "the rising American electorate" — are four times more likely to vote for a candidate who supports action on climate change. YPCC projects that in coming years these groups will make up a much larger segment of the total voting population.

"What we're hoping," Geoff Feinberg, Research Director at YCCC, told VICE News, "is that as the rising electorate grows, global warming will continue to rise up the list of campaign concerns."

Follow Shelby Kinney-Lang on Twitter: @ShelbKL