(CNN) A group of kids are suing the Trump administration over the climate emergency, which they say is being handled so negligently that it violates their constitutional rights.

And on Tuesday the feds tried yet again to stop the case from going to trial.

Calling the lawsuit "radical" and an "anathema," Jeffrey Bossert Clark, an assistant attorney general who represented BP after the 2010 oil disaster in the Gulf of Mexico, argued for the US government that the case should be thrown out in part because it is a "direct attack on the separation of powers" among the three branches of the federal government.

"We don't think there is a state-created danger here," he added.

Twenty-one young plaintiffs -- now somewhat less-young than when they filed the complaint in 2015 -- have watched their lawsuit pinball around the court system for years.

It now sits with a three-judge panel from the 9th US Circuit Court of Appeals. A decision on whether the case could advance toward trial is expected in the coming weeks or months.

The climate kids allege their constitutional rights to life, liberty and property are being violated by a US government that is knowingly promoting the extraction and burning of fossil fuels, which cause dangerous levels of warming, raising sea levels and promoting droughts and storms.

They also say they have a right to a safe atmosphere, and that they are being discriminated against as young people who will bear outsize consequences of the climate emergency.

The lawsuit has been called a moonshot attempt to get the federal government to act on climate, and has been compared to Brown v. Board of Education, which desegregated public schools. It's "the biggest case on the planet," University of Oregon law professor Mary Wood told CNN in 2016.

Along with a similar case in the Netherlands, Juliana v. United States is the "most promising" legal action on climate change in the world at the moment, said Michael Gerrard, director of the Sabin Center for Climate Change Law at Columbia University, who is not litigating the case.

"It's significant that in neither the (Dutch) nor the Juliana case have the government defendants contested the underlying science of climate change," Gerrard said. "Instead their defenses are what is the proper role of the courts, as opposed to the legislative and administrative branches dealing with the issue.

"It's also clear," Gerrard added, "that governments around the world have almost uniformly failed to act adequately" to address the climate crisis. Which is why people are looking to the courts.

Crowds assembled outside court in Portland, Oregon, in support of the youth. Demonstrators carried signs saying "Let the Youth Be Heard" and "The Atmosphere is a Public Trust."

Inside, Julia Olson, attorney for the young plaintiffs, argued that future generations will look back on the climate emergency as the defining legal challenge of this century.

"When our great-grandchildren look back on the 21st century they will see that government-sanctioned climate destruction was the constitutional issue of the century," Olson said. "We must be a nation that applies the rule of law to harmful government conduct that threatens the lives of our children so that they can grow up safe and free and pursue their happiness."

Olson asked the court to push the case toward trial, saying that the ultimate relief the young people seek is an order requiring the US government to create a plan to curb fossil fuel use and make the atmosphere safe and livable for young people and future generations.

Judge Andrew Hurwitz questioned whether the courts could do that.

"The issue here is whether this branch of government, embodied by the three of us today, has the ability to issue the relief that your clients seek," he told Olson from the bench. "We don't doubt that Congress or the President could give you the relief you seek without us."

"I don't think Congress or the President ever will without--" Olson said.

"Well, we may have the wrong Congress and the wrong President," Hurwitz said. "... The real question for us is whether or not we get to intervene because of it.

"You present compelling evidence that we have a real problem. You present compelling evidence that we have inaction. ... It may even rise to the level of criminal neglect. The tough question for me, and I suppose for my colleagues, is do we get to act because of that."

Clark, the assistant attorney general, said the case attempts to sidestep the usual procedures for creating federal laws, noting that it would have "earth-shattering consequences" for legal processes if it were successful. "Thurgood Marshall didn't bring one giant case to establish racial equality," he said. "It was an entire program, it was unfolded step by step."

Judges questioned whether that logic applies given that the enormity of the climate crisis is somewhat different from civil rights violations, which are illegal even on a small scale, too.

Much attention has focused on the case in part because of the dire stakes of the climate emergency, which have been clear for decades and yet continue to worsen.

A United Nations scientific body found last year that there were only about 12 years left to avert some of the worst effects of the climate crisis, including flooded coastal cities, deadlier heatwaves and the near-end of coral reefs. Global emissions of heat-trapping gases must be cut in half by 2030 and to net zero by mid-century, the group found in a landmark report.

Yet emissions continued to rise globally in 2018, according to the Global Carbon Project.

It's the failure of the US government to adequately address the climate crisis that makes this court action necessary, said Andrea Rodgers, senior attorney for Our Children's Trust, which represents the youth plaintiffs.

"What we've learned throughout history is that when there are systems that have been constructed purposefully (to violate) constitutional rights of citizens, you need all three branches of government to get involved and to remedy the constitutional violation," Rodgers told CNN.

Further delay of the case is dangerous for the planet and people, she said.

"Time is not our friend on this issue."