The Nebraska Supreme Court court just dropped the Keystone XL pipeline decision back onto John Kerry’s lap.

That means the secretary of state will have a chance to show just how committed he is to tackling climate change, environmentalists say, just a month after Kerry jetted to Peru to press for the world to act.

Activists say the former Massachusetts senator’s green credibility hangs in the balance.

“You can’t approve a pipeline from the most climate-intensive fuel source on the planet and be serious about dealing with climate change,” Sierra Club Executive Director Michael Brune said weeks before a ruling Friday from the Nebraska Supreme Court removed a major legal impediment to the project.

Asked if Kerry can salvage his climate resume if he endorses Keystone, 350.org co-founder and anti-Keystone activist Bill McKibben didn’t mince words. “No. Keystone’s obviously a keystone,” he said in an email. “Approve that and the rest is happy talk — you can’t cut carbon without cutting carbon.”

On the heels of President Barack Obama’s recent scoffing of the arguments of pipeline supporters, greens are more certain than ever that the project will never be approved.

But anti-Keystone activists — led by 350.org and groups like the Natural Resources Defense Council — have nonetheless launched a lengthy campaign to pressure Kerry, and ultimately Obama, to kill the pipeline. They’ve not only sought to undercut the merits of the project, but they’ve also appealed to Kerry’s soft spot: his crusade to take on climate change.

At December’s climate talks in Lima, dozens of activists organized anti-Keystone protests timed to coincide with Kerry’s brief visit. They warned that the United States’ new reputation as an international climate change leader, which reached new heights when the U.S. and China jointly announced a plan to curb carbon pollution in November, would be tarnished if the Obama administration green-lights the pipeline.

McKibben has said that being “willing to block a big project because of its effect on climate” would strengthen Obama’s hand during next December’s United Nations climate talks in Paris.

But for all of the talk about Keystone in North America, those closely tracking the climate talks said few international negotiators were talking about the pipeline during tense discussions behind closed doors.

“Honestly, it doesn’t come up much,” Todd Stern, the Obama administration’s top climate change negotiator, said in a recent interview. “That’s not to say that people from [non-government organizations] or the press aren’t going to say, ‘What do you think about Keystone?’ But if you mean in the world of the actual working with other governments, maybe once in a blue moon. But it’s pretty rare.”

Instead, negotiators are focused on ironing out the details of the sweeping climate agreement that countries aim to finish in Paris, which could include demands by poorer nations for billions of dollars in aid from rich polluters like the United States.

Jennifer Morgan, global director of the World Resources Institute’s climate program, agreed that Keystone wasn’t on delegates’ radar in Lima. But she nonetheless said a decision by Obama to approve Keystone would reverberate in international climate circles. “If Keystone were to move forward, it would cause countries to question whether the commitment of the Obama administration to climate change is completely solid,” she said.

Connie Hedegaard, the European Union’s former top climate change official, agreed that the Obama administration’s decision would echo around the globe.

“If the U.S. Administration decided against the Keystone XL pipeline it would send a very important political message to the rest of the world: That because you CAN do something to promote more fossil fuels — you can also for climate reasons deliberately decide NOT to do it,” she said in an email.

The State Department is in the midst of reviewing whether building the Canada-to-Texas oil pipeline would serve the U.S. national interest, although that process has been on hold since April because of the Nebraska court case. Kerry is supposed to make that decision, although Obama has promised to have the final word on whether the U.S. will green-light the pipeline.

Kerry has spent years urging the U.S. and world to combat climate change, a call he took up during frequent floor speeches in the Senate and in the 2009 and 2010 negotiations for a cap-and-trade bill. Those talks ultimately collapsed amid opposition from Republicans and some moderate Democrats.

By all accounts, Kerry, like Obama, views climate change as one of the defining issues of his career. Winning a strong, long-term global agreement in Paris next year would be one of his biggest accomplishments as secretary.

In his speech in Lima, Kerry called on wealthy nations to lead and urged developing nations to follow suit. He made a detailed case for transitioning to renewable energy and took aim at fossil fuels, arguing that they impose major long-term costs on the world.

“Coal and oil may be cheap ways to power an economy today, in the near term, but I urge nations around the world, the vast majority of whom are represented here at this conference, look further down the road,” he said. “I urge you to consider the real, actual, far-reaching costs that come along with what some think is the cheaper alternative. It’s not cheaper.”

While he never mentioned Keystone, climate activists took solace in his remarks.

“Secretary Kerry didn’t mention the pipeline, but he made all the right arguments against it,” 350.org spokesman Jamie Henn said. “He cited the global threat of climate change, the costs associated with dirty fuels, and the economic opportunity of a clean energy economy — and he called on citizens to make climate change an issue that elected officials cannot ignore. We’re doing our part, now he must do his.”

And the Sierra Club’s Brune said he’s confident Kerry will reject Keystone because of his long history advocating for action on climate change.

“Secretary Kerry has been nothing if not consistent throughout his political career,” he said. “Past behavior is a pretty good indication of future behavior.”

Kerry has avoided discussing Keystone during his time at State, hewing to the department’s mantra about not commenting on a project that is under review. He didn’t even take a clear position on the project when he was Senate Foreign Relations chairman — though he did vote against a Republican amendment in 2012 that would have circumvented the administration’s review process by forcing approval of the project.

Kerry’s focus on climate change goes back decades. He was a fixture at international climate change talks long before he became secretary of State.

The then-Massachusetts senator was so committed to attending climate negotiations in Bali in 2007 that he organized a quick trip around crucial Senate votes, spending more time on the overseas flight than he did on the ground. And when scheduling conflicts prevented Kerry from meeting with a key Chinese climate official during a 2009 trip to the country, he organized a last-minute huddle at the airport, according to former aides.

His first hearing as chairman of the Foreign Relations Committee focused on climate change and included testimony from Al Gore.

“I can tell you, that was absolutely a statement,” said Kathleen Frangione, who served as a top climate adviser for Kerry when he was in the Senate.

Obama, for his part, has said he wouldn’t approve the pipeline if it “significantly exacerbate[s] the problem of carbon pollution.” The president has been sending public signals, including during a news conferences last month, that he doesn’t buy many of the arguments of Keystone’s supporters, although he has not indicated what his final decision will be.

If Kerry and Obama decide to approve the project, they can point to the State Department’s own environmental assessments, which concluded that the pipeline would not dramatically worsen climate change. Industry supporters of the pipeline are making that case.

“The clear scientific consensus is that Keystone XL passes President Obama’s climate test. Experts in Secretary Kerry’s own State Department found, in five assessments in over six years, that the pipeline will have a minimal impact on the environment,” Katie Brown, a spokeswoman for the industry-backed pro-Keystone group Oil Sands Fact Check, said in a statement. “Will Secretary Kerry listen to them or a fringe group of activists who are desperately trying to gain traction for their radical anti-fossil fuel agenda?”

But plummeting oil prices could upend the State Department’s previous conclusions.