Bay Area billionaire Tom Steyer’s blitz against candidates who are soft on climate change is underway in seven states, but some prominent fact-checking groups say he’s emitting enough hot air to melt a few glaciers.

Negative reviews from watchdogs like PolitiFact, FactCheck.org and the Washington Post are dogging one of the nation’s biggest political donors, a former hedge fund manager who ditched his ties to fossil fuels and presented himself as a transparent antidote to the conservative Koch brothers’ semi-clandestine funding network.

Playing fast and loose with the facts is “unwise simply because you’re handing a bat to your opponents to use squarely over your noggin,” said Larry Sabato, director of the University of Virginia Center for Politics. “It sounds to me as if they need their own fact-checkers on staff.”

Steyer, 57, of San Francisco, was unavailable for an interview this week. But Bobby Whithorne, spokesman for Steyer’s NextGen Climate Action super PAC, said: “We stand by our ads.”

The Washington Post’s Fact Checker blog in January, however, awarded its dreaded “four Pinocchios” rating to a NextGen ad citing Chinese investment in Canada’s tar sands and claiming the controversial Keystone XL pipeline would produce oil only for other countries. The Chinese investment is small, the Post found, and NextGen took an oil executive’s words out of context to imply that no oil carried by the pipeline will remain in the U.S.

The ad “relies on speculation, not facts, to make insinuations and assertions not justified by the reality,” the Post said.

Last October, PolitiFact — a renowned fact-checking project run by the Tampa Bay Times — gave its “pants on fire” rating to a NextGen ad claiming Virginia Attorney General Ken Cuccinelli, then running for governor, wanted to “eliminate all forms of birth control.” Cuccinelli has repeatedly said he has no interest in restricting contraception, PolitiFact noted.

This month, PolitiFact gave “half-true” ratings to a pair of NextGen ads attacking Florida Gov. Rick Scott’s ties to energy companies and polluters. FactCheck.org, a project of the University of Pennsylvania’s Annenberg Public Policy Center, said it didn’t dispute the statement of critics that one of the Florida ads was “total fiction,” though the GOP response had “glaring factual problems” too.

And PolitiFact this month deemed “false” a NextGen ad accusing Iowa U.S. Senate candidate Joni Ernst of having signed a pledge that “protects tax breaks for companies that ship jobs overseas.” The pledge was a broad vow to oppose all tax hikes.

But Whithorne, Steyer’s spokesman, noted that an Iowa television station had determined that the ad was “mostly true.”

And he is quick to defend all of NextGen’s ads: “The facts are there, and we provide extensive backup to substantiate the claims. With less than 80 days until the midterms, we’ll continue to keep the pressure on the anti-science candidates and highlight their extreme positions.”

Steyer is used to political battles. He took lead roles in California ballot-measure campaigns to defend the state’s landmark greenhouse-gas emissions law and to close a $1-billion-per-year corporate tax loophole.

Two years ago, he announced he would step down from his business and turn to public policy. He founded NextGen and said he hoped to spend $100 million — half from his own pocket, half raised from others — to challenge candidates across the nation on climate-change issues. Steyer says the Koch brothers are out to enrich themselves, while he’s putting the planet first.

In Iowa, Ernst’s campaign is urging television stations to take down NextGen’s ad. In Florida, Scott’s legal counsel issued cease-and-desist letters telling stations to stop airing one of the ads; at least one, in Fort Myers, complied. And Californians Against Higher Oil Taxes, founded earlier this year by oil-industry trade groups and other business organizations, says Steyer’s ads help make a case against him.

“It just goes to show that if the public understands the truth about his policies, they’re not going to support it because it’s going to drive up their cost of living,” spokeswoman Sabrina Lockhart said. “So it seems he’s pivoted to lies and distortions of the truth to sell the public on something they’re not buying.”

Sabato, however, said the ads are meant more to mobilize already-sympathetic voters than to change minds. And Tom Hollihan, a University of Southern California political communications expert, agreed.

“For the people for whom the ads are the primary audience, the fact-checking might not have much consequence,” Hollihan said, adding that the fact checks have more effect in correcting the record for media and policymakers.

Steyer doesn’t seem to have raised enough money to reach his $100 million spending goal but has spent more than $20 million so far in this election cycle.

Dan Schnur, director of USC’s Unruh Institute of Politics and former chairman of California’s Fair Political Practices Commission, said the ads’ targets have little choice but to up their own antes.

“Until someone decides to spend just as much money in opposition to these messages as NextGen has spent broadcasting them,” he said, “most voters will never hear a doubting word.”