Federal appeals court judges went after both the Obama administration and its opponents equally on Tuesday, in a whirlwind series of oral arguments meant to suss out the centerpiece of the president's climate change agenda, the Clean Power Plan.

The regulation would set carbon emission reduction goals from existing coal power plants for each state, leaving it up to them on how to reach those goals. The rule aims to reduce carbon pollution from the power sector by 30 percent by 2030.

Judge Thomas Griffith began the charge Tuesday morning at the D.C. Circuit Court of Appeals by hammering at states' arguments that the plan would cause a monumental shift in how they generate and receive power.

"How will it be transformational?" Griffith asked West Virginia Solicitor General Elbert Lin. He added that from where he stood, the shift in coal use "hardly sounds transformative."

In previous cases before the court, millions of sources have been potentially at risk, Griffith said. The Clean Power Plan case features "marginal differences" between its effect on power plants with or without it going into effect.

Judge David Tatel then chimed in, saying the "only thing that sounds transformative" is the Environmental Protection Agency regulating carbon dioxide. "But the Supreme Court dealt with that" years ago. The Supreme Court ruled a decade ago that EPA could regulate carbon pollution under the Clean Air Act.

Lin argued that what EPA is doing under the Clean Power Plan is fundamentally different from how it has used its authority in the past. It's not about the agency regulating carbon. It's about how it regulates it, Lin said.

He added that if Congress wanted the agency to apply its authority this way, it would have done so in the law.

Lin and the EPA faced 10 judges on the D.C. Circuit Court of Appeals in an unprecedented en banc review by the full court, instead of a normal three-judge panel.

Judges were just as hard on Department of Justice attorneys defending the EPA. Judge Brett Kavanaugh led the questioning on EPA's defense over its use of authority to direct states to move forward.

Kavanaugh said the EPA plan raises questions about the "separation of powers" between Congress and the executive branch. He said for any major regulations, "Congress has to specifically authorize" this type of action.

"This is a huge case," Kavanaugh said. "It has huge political and economic implications." He said for EPA to convince him otherwise, "you have to show it is not a case of political and economic significance."

Eric Hostetler, representing the EPA, said the rule requires no new authority from Congress and would cut carbon dioxide emissions "cost effectively" with no effect on grid reliability.

Kavanaugh said "I don't swallow" EPA's argument. "It is fundamentally transforming the industry" by forcing it to pay a "huge financial penalty."

Tatel added to Kavanaugh's questions by asking why the EPA doesn't think Congress needs to step in.

But not giving Hostetler too much breathing room, Judge Janice Brown said she thought the administration was touting the rule's "transformative" aspects.

Hostetler said the challengers take much of the administration's description of the rule out of context, pointing out that Brown was taking her line of questioning from a single bullet point in a White House fact sheet.

He explained that what the Obama administration means by "transformative" is already happening in the electricity markets with the shift from coal to more low-emitting natural gas and renewable energy growth.

Kavanaugh responded: "Congress should be making the big policy decisions. And this is an ongoing debate in Congress." He said he understands the frustrations of the White House, which failed to pass a climate change law on Capitol Hill, but the separation of powers are clear.

Griffith asked why the debate over regulating carbon pollution isn't happening on the floor of the Senate and instead is being decided by "un-elected judges."

The hearing was attended by EPA Administrator Gina McCarthy and senior staff from both the agency and the White House.

Oral arguments are expected to continue through Tuesday afternoon.