Environmental Protection Agency Administrator Lisa P. Jackson maintained a poker face as she spoke at a congressional budget hearing last week.

As the Obama administration’s point person for environmental regulations, Jackson received her seventh grilling on Capitol Hill this month, more than any other federal agency director has faced, according to committee and agency staffs.

At Friday’s joint hearing of two House Energy and Commerce Committee subcommittees, Jackson addressed familiar questions, most of them from Republican lawmakers. How would you describe carbon? “As black carbon soot,” Jackson answered in part. Is the EPA reviewing farms and particulate-matter pollution that can be kicked up in dust? Not really, Jackson said. “EPA recognizes that dust happens.”

House Republicans have been challenging the EPA administrator on whether carbon is a pollutant, in hearings aimed at cutting the agency’s budget and curbing its ability to regulate greenhouse gases. They say the EPA proposed greenhouse gas and other regulations without determining the extent of costs and job losses in manufacturing.

The objections also amount to a challenge of a Supreme Court ruling nearly four years ago that rebuked the Bush administration for failing to do what the EPA is now attempting.

In that case, Massachusetts v. EPA, the Bush administration claimed that it lacked the authority under the Clean Air Act to determine whether greenhouse gas emissions from automobiles endangered public health, and declared that it would not do so for economic reasons, even if it had the authority.

The court ruled 5 to 4 against the administration in April 2007, saying the EPA not only had that authority under the Clean Air Act but also was obligated to base its reason for not studying greenhouse gas pollutants solely on science, not economic concerns.

After the Obama administration announced plans to regulate the largest greenhouse gas polluters, House Republicans drafted and passed a bill that would eliminate nearly a third of the EPA’s $9 billion budget — the largest percentage cut of any federal agency and the deepest EPA budget cut in 30 years, according to records at the Office of Management and Budget.

In the unlikely event that it passed both houses of Congress, the measure could overturn the agency’s plan to reduce pollution under the Clean Air Act and the Clean Water Act. Senate Democrats have vowed that it will not.

“I was in EPA for 20 years. I’ve never seen anything quite like this,” said Eileen Claussen, president of the Pew Center on Global Climate Change.

Claussen’s complaint echoed those of Democrats such as Sen. Barbara Boxer of California, who has invoked the court decision while supporting the EPA and Jackson in budget hearings and said that “Republicans have launched a breathtaking attack on public health on behalf of the biggest polluters in America.”

“The Congress can always strip an agency of the power that it gave them,” said Jody Freeman, founding director of the Harvard Law School Environmental Law and Policy Program.

Freeman, who served in the Obama White House in 2009 and 2010 as counselor for energy and climate change, said the agency has been careful to target the biggest polluters — and not small businesses — with regulations. But Congress sets the laws, and Republicans are well within their rights to alter the EPA’s goals as they see fit.

“You obviously need both chambers to vote on it, and the president needs to sign it,” Freeman said.

The budget bill is loaded with provisions called riders that would ban the agency from regulating water pollution in the Chesapeake Bay and the Mississippi River, take away its ability to veto state permits for development that often leads to pollution, and bar it from acting to reducing particulate matter pollution that affects health when it’s breathed into the lungs and works its way into the bloodstream.

Republicans say that studies such as one by two manufacturers’ groups projected that 7 million jobs would be lost in the decade beginning in 2020 if their client organizations are forced to pay up to $1 trillion to meet the EPA’s ozone standards, said Alicia Meads, director of energy and resource policy for the National Association of Manufacturers. Meads also cited a study by the Council for Industrial Boiler Owners that said 16,000 jobs would be lost for every $1 billion spent to comply with EPA boiler regulations.

“We consider it an overreach,” Meads said. “This administration has been extremely aggressive in environmental regulations, and it’s very hard for our members to keep up with them.”

Nonetheless, David Doniger, policy director for the Natural Resources Defense Council’s climate center, argues that the Supreme Court ruled correctly in saying the EPA was responsible for regulating pollutants under the Clean Air Act: “Here’s the set of pollutants. Go out and curb them.”

In the eyes of many conservatives, however, the court got it wrong, said Diane Katz, a fellow who studies the EPA at the Heritage Foundation.

“I think there’s a significant amount of people . . . concerned about the effects on our economy, and the engine of our economy, that being energy. I can say it’s probably pretty common that people don’t feel [global warming] is an immediate threat.

“There’s no strong consensus on whether carbon dioxide causes global warming or climate change. The court’s ruling was based on a conclusion that such a consensus exists,” Katz said.

“That’s just total nonsense,” Doniger said. “There’s never been a scientific finding by the EPA better documented and better supported than the one that regulates greenhouse gases.”

As Jackson has learned, the Supreme Court ruling has been of little help in a debate that oscillates back and forth. By week’s end, her appearances on the committee hotseat more than tripled those of the energy secretary and nearly doubled those of the interior secretary in March, according to their staffs.

The secretary of health and human services, charged with implementing the controversial new health-care law, has appeared only once, a spokesman said.

At Friday’s hearing of the subcommittee on energy and power and the subcommittee on environment and the economy, Jackson worked hard to remain stoic as questions, sometimes odd, rained down.

“You used in your opening statement the term ‘carbon pollution.’ Would you describe that?” asked Rep. Joe Barton(R-Tex.). “That table in front of you is carbon. Carbon itself is not a pollutant.”

Jackson said she was referring to carbons that clearly are pollutants, such as those in emissions that flow from the smokestacks of power plants, cement-mixing facilities and oil refineries in Barton’s state.

“Let’s talk about mercury,” Barton said. He criticized Jackson for saying that plants emit “tons and tons” of it. “You might want to check your facts on that. It’s not tons a year but pounds,” Barton said.

Yes, Jackson agreed, it was pounds. “Two thousand pounds equals a ton,” she said, finally cracking a smile.