Expert witnesses had already booked their travel to testify in the long-awaited landmark climate change case. Some of the plaintiffs -- young activists who are suing the federal government -- altered their school schedules in order to live in Eugene, Ore., during what was expected to be a first-of-its-kind trial.

But thanks to an eleventh-hour stay by the Supreme Court, those plans are now in doubt.

An order late last week from Chief Justice John G. Roberts Jr. handed a notable, if temporary, victory to the federal government, which has for years tried unsuccessfully to halt a landmark suit by 21 young people who argue that the failure of U.S. leaders to combat climate change violates their constitutional right to a clean environment. The case was scheduled to be heard beginning on Oct. 29 in a U.S. District Court in Oregon, but the administration asked the Supreme Court to intervene.

“It is extremely rare, if not unprecedented, for the Supreme Court to enjoin a trial when the Court of Appeals is still considering the case. Ordinarily they’ll wait for the lower court to rule,” said Michael Gerrard, an environmental law professor at Columbia University.

The plaintiffs in the case range in age from 11 to 22. The goal of their lawsuit is to compel the government to scale back its support for fossil fuel extraction and production and to support policies aimed at reducing the greenhouse gas emissions that contribute to global warming.

“It’s one of the most profound threats to young people I’ve ever seen,” Julia Olson, the attorney for the youths and executive director of Our Children’s Trust, said of climate change in an interview with The Washington Post last week. She added that the only way for the nation to make a systemic shift from its reliance on fossil fuels is for the government to lead the way through policy action.

“It’s not going to happen otherwise,” she said.

Both the Obama and Trump administrations have repeatedly asked lower courts to halt the lawsuit since it was filed in 2015, questioning the merits of the case, saying discovery requests were “burdensome" and arguing that the suit would usurp the authorities of Congress and federal agencies.

Time and again, lower courts have allowed the case to proceed. The government also went to the Supreme Court earlier this summer seeking a stay, but the court in an unsigned opinion also refused to stop the case, calling the request “premature.”

But the court also foreshadowed it might be willing to step in at some point: “The breadth of respondents' claims is striking, however, and the justiciability of those claims presents substantial grounds for difference of opinion. The District Court should take these concerns into account in assessing the burdens of trial and discovery."

Last week, with the landmark trial looming, the Trump administration once again asked the Supreme Court to intervene.

“This suit is an attempt to redirect federal environmental and energy policies through the courts rather than through the political process, by asserting a new and unsupported fundamental due process right to certain climate condition,” Department of Justice lawyers wrote, adding that a district court “has allowed this improper suit to proceed for nearly three years over the repeated objections of the government."

Late Friday, Roberts temporarily halted the case, pending a response from lawyers for the children. The decision angered environmental activists who have been eager to see the long-awaited trial begin, but others welcomed the delay.

“The Supreme Court is acknowledging what we have really known all along: this is a policy issue that belongs in the halls of Congress, not the courts," Jeff Holmstead, who headed EPA’s air and radiation office under President George W. Bush and is now a lawyer representing energy firms, said in a statement. He added, "These young activists' considerable time and effort would be better spent working to influence the federal and state legislative debates surrounding policy solutions that address the challenges of climate change in an effective and meaningful way.”

The plaintiffs in the case show no signs of going away quietly.

On Monday, their attorneys filed a 103-page response to the Supreme Court. It argued that the Trump administration had not met the legal bar for a stay and would not suffer “irreparable” harm in having to go to trial.

“This case clearly poses profoundly important constitutional questions, including questions about individual liberty and standing, the answers to which depend upon the full evaluation of evidence at trial,” the lawyers wrote, adding, “These young plaintiffs, mere children and youth, are already suffering irreparable harm which worsens as each day passes with more carbon dioxide accumulating in the atmosphere and oceans."

Dino Grandoni and Chris Mooney contributed to this report.