When Hansen, the director of the Goddard Institute for Space Studies, showed up at a briefing on Capitol Hill yesterday, it was with little fanfare and minus the presence of national television networks.

The man termed "the father of global warming" has irked many longtime supporters with his scathing attacks against President Obama's plan for a cap-and-trade system. Now, a leading Republican climate skeptic is considering calling Hansen as a witness at upcoming Senate hearings. A House Democrat, meanwhile, labeled Hansen's Capitol Hill appearance yesterday "irrelevant." With landmark climate legislation heading to the Senate after passage in the House last month, the friction surrounding Hansen raises questions about what role, if any, the Iowa-born scientist will play in the upcoming debate.

"He's losing some of his political luster with Democrats," said Kenneth Green, a resident scholar at the American Enterprise Institute, a conservative think tank. "When he goes off the reservation and makes loopy comments, they probably cringe and hope no one will see them."

Hansen, for his part, said he believes "some Democrats deserve to be criticized." Speaking in a room in the Capitol Visitor Center, Hansen repeated his unwavering support for a carbon tax as the only way to lead the United States and the world out of a climate catastrophe. The House-passed bill sponsored by Reps. Henry Waxman (D-Calif.) and Edward Markey (D-Mass.) is riddled with too many giveaways to the coal industry, he said.

"I'm connecting the dots between the science and the policy. If I don't do that, then the special interests do it and they screw it up," Hansen said. Sponsored by Friends of the Earth -- one of the few environmental groups publicly critical of the Waxman-Markey bill -- the briefing drew about 100 people, including Senate staffers. It featured carbon tax supporters like Robert Shapiro, a former Clinton undersecretary of Commerce.

After the talk, a handful of admirers swarmed around Hansen, while one tried to hawk a product to sell to NASA.

Under attack from the left and right

The House bill, which is supported by much of the Democratic leadership, backs the cap-and-trade concept. Instead of a tax, it places an overall ceiling on greenhouse gas output and requires businesses to buy and sell carbon allowances in a marketplace to meet their emission cuts.

In a just-published piece in the Huffington Post, Hansen called the House legislation a "Ponzi-like" scheme and outlined its "egregious flaws" in bullet-point form. Those deficiencies, in his view, include an over-reliance on carbon offsets, which allow emitters to meet emission cuts by buying credits outside their own factories.

He also thinks there needs to be greater research and deployment of nuclear power, because that's the only way to "get China and India off coal."

Hansen's backers argue that politicians ignore his political suggestions at their own peril. They say Hansen's science was attacked for years only to be proved right, and that the same is true with his legislative suggestions.

"Mr. Hansen certainly proved ahead of his time with his modeling of global climate change. Is [he] also ahead of his time with his call for a carbon tax?" wrote one blogger at the Wall Street Journal.

Yet some worry that the climate scientist is being used as a political attack tool by conservatives who don't even believe in his computer models.

"The right wing loves what he's doing," said Joseph Romm, a senior fellow for the Center for American Progress, a think tank with ties to the Obama administration.

Sen. James Inhofe (R-Okla.), who once called global warming a "hoax," recently cited Hansen's criticisms of the House legislation. During the House floor debate, some GOP members voting against the bill mentioned Hansen.

Despite policy differences, still educating people on the science