This post was co-published with TIME.

When a question about the Regional Greenhouse Gas Initiative (RGGI) came up in a New Hampshire gubernatorial debate in September, the candidates' answers hardly mattered. Even before a word had been spoken, the big winner was Americans for Prosperity (AFP), the Koch-brothers-backed group that has made the fight against the controversial regional cap-and-trade initiative into a crusade.

The group's New Hampshire arm has funded two years' worth of robocalls, mailers and radio ads to try to persuade Granite State voters to end the state's participation in the program, which AFP views as a tax on energy. Though less eye-catching than the tens of millions of dollars AFP has poured into the presidential race, the group's effort in New Hampshire shows how a little bit of dark money can go a long way in state-level fights. Nonprofits funded by anonymous donations have swooped in to finance ballot measures and state races around the country in 2012. The Iowa-based American Future Fund gave $4 million to support a California proposition that would limit labor unions' political fundraising. The American Federation for Children, a Washington, D.C., pro-school-voucher group, paid for mailers and ran ads supporting friendly state-legislature candidates in Florida and Wisconsin.

It's unknown how much AFP is spending in New Hampshire, as such expenditures do not have to be reported under the state's campaign-finance laws. But if AFP will not say how much it has spent trying to kill RGGI, it is not shy about taking credit for putting the program front and center at polling places this week.

"This is an issue being discussed in the gubernatorial debate," Corey Lewandowski, AFP's New Hampshire state director, says. "Our education campaign has made a major impact."

New Hampshire and eight other states participate in RGGI, a regional agreement under which limits are placed on carbon emissions at power plants and emission allowances are auctioned to utilities. States have used the proceeds from those sales to pay for energy-efficiency programs since 2008. But the program is opposed by conservative groups that believe the threat of climate change is overstated. This summer, the state legislature in Concord passed an AFP-supported bill requiring New Hampshire to withdraw from RGGI if two other New England states pull out. Last year, following another AFP campaign, New Jersey Gov. Chris Christie withdrew from RGGI.

AFP has helped make RGGI's future a significant issue in the New Hampshire governor's race. Republican Ovide Lamontagne, crowned "conservative of the year" by AFP in 2011, promises repeal. At the September debate, Democrat Maggie Hassan, who helped pass RGGI as a state senator, said, "I was proud to be a sponsor of that tax — uh — the energy-efficiency program."

AFP pounced. A radio ad from the group repeatedly plays the audio of Hassan calling the program "that tax."

Days before the election, the race is a dead heat.

For more on Dark Money, read Dark Money Rises.