Billionaire climate activist Tom Steyer's NextGen Climate Action PAC is targeting Iowa GOP Senate candidate Joni Ernst in a $2.6 million TV ad campaign.

The ad rolled out Wednesday is the first in a series of spots against Ernst, who is trying to best Democratic Rep. Bruce Braley for the state's open seat. A CBS News/New York Times/YouGov poll of likely voters released Monday showed Ernst up on Braley, 48 to 47.

The first commercial slams Ernst for signing the Americans for Tax Reform pledge to refuse raising taxes. The group said signing "The Pledge," as it's known, amounts to "eliminat[ing] support for nearly 80,000 renewable energy jobs in Iowa," a reference to expired wind and biodiesel industry tax credits that Democrats and some Republicans are seeking to revive.

Other ads, which will run over five weeks in Des Moines, Cedar Rapids and the Quad Cities, will seek to tie Ernst to the oil industry and the conservative billionaire Koch brothers, said Suzanne Henkels, a spokeswoman with NextGen Climate.

Ernst campaign spokeswoman Gretchen Hamel told the Washington Examiner that the push "proves Bruce Braley and his allies are desperate."

"Unlike Bruce Braley who flip flops to suck up to billionaire environmental extremists, Joni is an independent leader who isn’t afraid to stand up for what she believes, even when people don’t agree with her," she said.

Ernst has come under fire from left-leaning groups for attending a Washington fundraiser ExxonMobil and the American Petroleum Institute are throwing in her honor on Wednesday.

API has led the charge to repeal the federal renewable fuel standard — which sets a mandate for the amount of biofuel that must be blended into gasoline by 2022 — that is key for farmers and biofuel companies in Iowa, which produces more biofuels than any other state.

Ernst, a state senator, has said she supports the renewable fuel standard and would eliminate it only if subsidies were removed for all energy sectors, saying that she is "philosophically opposed" to such incentives.