Groups sponsoring an upcoming Heartland Institute conference are standing by the conservative think tank amid a controversy surrounding a short-lived billboard campaign linking belief in climate change to the Unabomber and Charles Manson.

The Heartland Institute ended the Chicago-area billboard campaign last week amid a backlash over the ads. The Association of Bermuda Insurers and Reinsurers, for example, wrote to Heartland Institute President Joseph Bast last week to “disavow any future relationship with your organization.”

{mosads}But while the Heartland Institute’s corporate sponors are under pressure to cut ties with the think tank, groups that are sponsoring Heartland’s May 21-23 climate change conference in Chicago said they have no intention of pulling their support.

“CEI is not withdrawing as a co-sponsor of the International Conference on Climate Change,” Myron Ebell, director of the Center for Energy and Environment at the Competitive Enterprise Institute, said in an email.

“This is not an endorsement of Heartland’s ads, but I would point out to you that the reason they may have thought they were appropriate is that the media have been so tolerant of similar attacks by the left on climate realists.”

Ebell said environmental groups have aggressively attacked skeptics of climate science.

“There have been only a few mild objections made to the use of the word denier by the media. Nor have there been outcries when prominent environmentalists have called for war-crime-type trials,” Ebell said. “It seems to me that Heartland may have thought that they were merely responding in kind.”

Reason Foundation Vice President Julian Morris also said the group would continue to sponsor the conference.

“We disagree with Heartland’s ad campaign, but still plan to co-sponsor and participate in their conference because Reason believes open debate and the exchange of ideas are vital to a free society,” he said in a statement.

Steve Milloy — the publisher of Junk Science, a website devoted to questioning climate science that is sponsoring the conference — stood by the advertisements.

“Heartland’s ad campaign was fact-based and provocative,” Milloy said in an email, arguing there is no “backlash” against advertisements by groups like the American Lung Association that say GOP lawmakers are threatening public health by opposing Environmental Protection Agency regulations.

John Humphreys, head of the Australian Libertarian Society, another group sponsoring the conference, said the Heartland Institute campaign was in “poor taste.” But he said the group would still sponsor the conference.

“Now that they have been discontinued we look forward to an interesting conference that will hopefully present a range of views,” Humphreys said.

The Heartland Institute touted a billboard campaign last week that featured a photo of Ted Kaczynski, known as the Unabomber, that stated, “I still believe in global warming, do you?”

The group had said that the campaign would also feature Charles Manson, Fidel Castro and perhaps Osama bin Laden asking the same question. The campaign prompted swift condemnations from environmental groups and others.

The billboard unleashed a social media-fed campaign, including a petition from the advocacy group Forecast the Facts calling on Heartland’s corporate backers — including Microsoft and Pfizer — to immediately pull their funding.

Heartland disputes the view held by the overwhelming majority of scientists that the planet is warming and human activities — including burning fossil fuels — are playing a major role.

A small minority of scientists call data and modeling on warming trends and the human contribution inaccurate or inconclusive.

Ben Geman contributed to this story.