When a group of more than 20 protesters showed up in the halls of the U.S. Senate on a recent February day, they would have been forgiven for expecting a chilly reception. For the past seven months, sit-ins at a range of offices–from California Governor Jerry Brown’s to Speaker of the House Nancy Pelosi’s–had followed a similar pattern: show up, sing songs, get led away in handcuffs for disrupting the peace. But on that particular Wednesday, things were different.

Instead of being dismissed or arrested, this band of environmental activists from a group known as the Sunrise Movement was warmly welcomed. Democratic Senators’ aides applauded their songs, led them to back offices for meetings and cheered their efforts. “It starts with what you’re doing, from the bottom on up,” Vermont Senator Bernie Sanders told them. “I just want to thank you.” In the weeks that followed, Senate minority leader Chuck Schumer, after years of near radio silence on climate change, gave a series of speeches on the chamber floor. “For the first time in a long time, the Senate is finally debating the issue of climate change, and if you ask me, it’s about time,” he said. “Climate change is an urgent crisis and an existential threat.”

It’s not just Democrats who suddenly want to focus on climate change. President Trump seized the opportunity to double down on his denial of climate science, while other Republicans began recalibrating their messaging. Florida Congressman Matt Gaetz, an ardent defender of the President who introduced a bill in 2017 to eliminate the EPA, responded to Trump by tweeting, “Climate change is real.” In December, John Cornyn of Texas, who until recently served in GOP Senate leadership, tweeted positively about a tax on carbon emissions, and a month later, Republican Representative Francis Rooney of Florida and Democratic colleagues joined to introduce a carbon-tax measure in the House.

This shift has been a long time coming. Scientists have understood for decades that climate change is happening and that humans are causing it; recent studies, including a landmark report in October from the U.N., have shown that things are even worse than we thought. Global temperatures have already risen 1°C since the Industrial Revolution; if the planet heats by much more than an additional half a degree, we could see some of the most catastrophic effects of climate change, including the death of the world’s coral reefs and the inundation of entire island nations.

That reality has resonated with the public: more than 70% of Americans now understand that climate change is taking place, according to data from the Yale Program on Climate Change Communication. A February NBC News/Wall Street Journal survey found that two-thirds of Republicans believe their party is “outside the mainstream” on the issue.

Into this new political reality came the Green New Deal–equal parts policy proposal and battle cry. The resolution, introduced by Representative Alexandria Ocasio-Cortez of New York and Senator Ed Markey of Massachusetts, calls for the U.S. to launch a broad “mobilization” to decarbonize the economy while tackling a slew of other social ills. The response was mixed. People loved it. People loathed it. Others were confused by it. But in D.C., where climate has long been relegated to third-tier status, lawmakers could no longer avoid the issue.

Sunrise Movement activists call for a Green New Deal on Capitol Hill Aurora Samperio—NurPhoto/Getty Images

Within weeks of the proposal’s release, Democrats competed to burnish their green credentials. Nearly every Democratic candidate for the 2020 presidential nomination has endorsed the Green New Deal. Washington Governor Jay Inslee entered the race on a climate-themed campaign–something unthinkable just a few years ago, when Hillary Clinton and Donald Trump didn’t field a single question about climate change in their presidential debates.

Some Republicans scrambled to counter what they saw as a liberal threat: if they didn’t come up with a viable climate position, at least one they could point to rhetorically, they risked further ceding the issue to the Democrats, whose proposal they decry as socialist overreach. Behind the scenes, Republicans gathered in working groups trying to grasp for a solution. Major corporations–including in the oil-and-gas industry–pulled out their checkbooks to support conservative climate measures.

Love the Green New Deal or hate it, the conversation it has unleashed represents a shift in the discussion surrounding climate policy in the U.S., with ripples that will spread across the globe. The outcome of the debate will go a long way toward determining if humanity can avoid the most catastrophic consequences of a rapidly warming world.

Even before Ocasio-Cortez released her Green New Deal resolution, critics had begun to scrutinize the program, using every detail as a chance to condemn it. A congressional newcomer, Ocasio-Cortez has developed a reputation for taking critics and their talking points head-on, but in a recent interview with TIME she rejected the idea that she should have to defend the particulars.

“It’s a statement. It’s a vision document. And people want to pick it apart to death,” she says, agreeing with those who liken her proposal to the “bold persistent experimentation” that President Franklin Roosevelt advocated to end the Great Depression. “I hope that we start to get to more of an experimental spirit in government,” she says.

The Green New Deal experiments indeed, calling for everything from massive government spending on clean-energy research and development to cleaning up polluted industrial sites, all in service of quickly weaning the U.S. off greenhouse-gas emissions and helping vulnerable communities. The pieces of the package seem jumbled, but they are built on a rich history of ideas. Thomas Friedman first coined the term Green New Deal in a 2007 New York Times column and described the program as government “seeding basic research, providing loan guarantees where needed and setting standards, taxes and incentives.” Van Jones, the activist and political commentator, published The Green Collar Economy in 2008, about solving inequality and climate change at the same time. Chapter 4, titled “The Green New Deal,” outlined his vision for a program that would “birth a just and green economy.”

Around the same time, Inslee, then a Congressman, wrote Apollo’s Fire, highlighting stories of Americans’ benefiting from clean energy and calling for a moon-shot-like program to invest in a green economy. These ideas were so popular that Senators Barack Obama and John McCain pushed for green jobs in their 2008 presidential campaigns, and a handful of states have adopted some Green New Deal policies in recent years.

In California’s agricultural region, for example, former Fresno mayor Ashley Swearengin, a Republican, initiated a comprehensive plan to establish new transit options and redevelop the local economy while reducing greenhouse-gas emissions by 40%. The state as a whole has a plan to achieve 100% carbon-free electricity, a vision that’s gaining adherents in cities big and small and in a range of states from New Mexico to New York. “It’s not radical. By no stretch of imagination,” says Kevin de León, a Democrat who spearheaded many of California’s climate initiatives as president pro tempore of the state senate. California’s GDP is larger than that of all but four countries and its economy continues to thrive, he says, dismissing the critique that environmental reform would kill economic growth.

Still, the Green New Deal faces pushback not just from the political right but also from labor–a longtime stalwart of the Democratic Party. In early March, the AFL-CIO published a stinging critique of the proposal, calling it unrealistic and a threat to “members’ jobs and their families’ standard of living.” The oil-and-gas industry in particular supports millions of jobs, and while the text of the Green New Deal calls for a “just transition” to other industries, it offers few details. Republicans latched on to a talking point suggesting that the proposal would turn the U.S. into another Venezuela, pointing to the resolution’s inclusion of a job guarantee and universal health care, goals that many Democrats agree have no place in a climate package.

But if this ambitious climate plan seemed likely to wrench lawmakers further apart, it may actually do the opposite. In an era of festering dysfunction in Congress, all the green talk has actually sprouted green shoots, encouraging a national discussion around climate and, improbably, creating an opportunity to push real legislation. “If you care about moving the solution up the agenda, you have to salute what’s been accomplished here,” says Eric Pooley, a senior vice president at the Environmental Defense Fund, which has not endorsed the Green New Deal. “The fact that there are different points of view on different policy instruments is healthy.”

A carbon tax–once anathema to the right–is an unlikely beneficiary of this environmental glasnost, gaining support on both sides of the aisle. “If your goal is to reduce carbon emissions, a carbon tax will do that,” says Congressman John Delaney, a Maryland Democrat who is running for President and has introduced carbon-tax legislation. “It has an opportunity to get pretty broad support, including bipartisan support.”

Carlos Curbelo, a former GOP Florida Congressman who has led efforts for a carbon tax, said the Green New Deal offers a useful political foil. “It’s going to give Republicans and conservatives something that they can clearly oppose, which is always appreciated by the right,” he says. At the same time, he adds, Republicans are aware that public opinion is shifting. “It’s not going to be enough for a lot of members to say the Green New Deal is a massive socialist program. The next question is, What’s your solution?”

Democrats, of course, are happy to press that question. With the 2020 presidential campaign already under way, candidates are painting themselves as climate warriors. Nearly all of the dozen-plus Democratic hopefuls have endorsed a version of the Green New Deal, touted their environmental cred and described global warming as an existential threat.

But activists say most Democratic presidential candidates have a lot of catching up to do: while the party embraces climate science, that’s not the same as proactively fighting to halt dangerous warming. For now, few campaigns are proposing more than restoring Obama-era rules like the Clean Power Plan and recommitting the U.S. to the Paris Agreement.

Senator Elizabeth Warren, who has a 99% voting record from the League of Conservation Voters (LCV), has portrayed herself as the policy wonk among the presidential candidates, but she has introduced only one piece of climate legislation in her six years in the Senate, and her office lacks a dedicated climate staffer. Cory Booker and Kirsten Gillibrand, who both serve on the Senate Environment and Public Works Committee, have drafted a flurry of bills to deal with everything from toxic chemicals to cleaning up polluted sites, but neither has introduced comprehensive legislation to reduce emissions. Beto O’Rourke has come under fire for taking donations from people in the oil-and-gas industry. Kamala Harris, who joined the Senate in 2017, has a 100% score from the LCV but hasn’t proposed any significant climate legislation.

There are exceptions. Sanders, who has a long record of introducing climate legislation big and small in the Senate, plans to unveil a broad environmental plan. It is expected to include massive investment in infrastructure, the elimination of fossil-fuel subsidies and a ban on fossil-fuel extraction on public lands. Inslee and Delaney have both promised big on climate. Inslee says the issue will be a centerpiece of his presidency, while Delaney says a carbon-tax proposal could be enacted in his first 100 days.

But both Inslee and Delaney have thus far attracted only minimal support in presidential polls, and many Democrats fear that climate policy will fade as an issue if it isn’t a focus on the campaign trail. Indeed, many environmental leaders are still upset at President Obama for allowing cap-and-trade legislation to die in the Senate in 2009 after it passed the House. “When Democratic leaders see climate as a No. 7 issue,” says Senator Sheldon Whitehouse, a Rhode Island Democrat who has delivered more than 200 climate speeches on the Senate floor, “it stays a No. 7 issue.”

Behind the scenes, the energy surrounding the Green New Deal debate in D.C. has pushed top national Democrats to develop a policy that is both ambitious and workable. Before the current session of Congress began, House Speaker Pelosi responded to demands for a Green New Deal by creating a House select committee charged with laying the ground for climate legislation. In recent weeks, a group of Democratic staffers have convened in regular meetings led by Hawaii Senator Brian Schatz’s office to discuss a path forward on climate legislation. In the meantime, some members of Congress may introduce bits of Green New Deal–related legislation that they think could actually pass Congress now, like a proposal from Sanders on water infrastructure.

Congressional Republicans, feeling the shifting politics, have begun exploring policies that could answer the Democrats. GOP Senators Lindsey Graham of South Carolina and Cory Gardner of Colorado, both of whom are up for re-election next year, established a group for Republican members of Congress in February called the Roosevelt Conservation Caucus that looks for “market-based approaches” to environmental problems. Other GOP lawmakers have convened behind closed doors to discuss what ideas they can bring to the table. That’s a tall order for a party whose officials have ignored or denied the issue in recent years, and it remains to be seen whether the efforts will drive any wholesale change.

The private sector may help. In recent years, major oil-and-gas companies have slowly drifted away from long-standing opposition to climate policy. For an industry that thinks in decades-long time frames, federal legislation to combat climate change now seems inevitable, and industry leaders would rather see a conservative approach than a paradigm-shifting program like the Green New Deal. In recent months, a coalition of corporations has announced plans to spend millions of dollars lobbying for carbon-tax legislation paired with regulatory repeal and protection from climate-related lawsuits. For moderate lawmakers, it’s becoming possible to imagine a grand compromise that could include some elements geared for both sides of the aisle, say, investing carbon-tax revenue in clean-energy research and development, with a tax cut elsewhere and support for ailing coal communities.

But any compromise would face the scrutiny of the activists who at last have helped make climate change a front-of-mind issue. Advocacy groups have promised to ensure that Democrats not only continue to talk about the environment but also commit to bold action. Piecemeal commitments from Democratic lawmakers will not suffice, they say, and weak measures from Republicans should be rejected out of hand. “There are people talking about how a Green New Deal is improving the tax code or people saying that putting a climate provision into the infrastructure package is the Green New Deal,” says Varshini Prakash, executive director of the Sunrise Movement. “All of those things are good to do; they’re not the Green New Deal.”

Where does this leave all of us and the planet we inhabit? Even thinking optimistically, enacting climate-change legislation that matches the scale of the challenge is hard to imagine: the science is damning and the clock is ticking. And while the conversation may be shifting, there’s still a climate-change-denying President in the White House and nothing like a broad consensus on a path forward among Democrats.

In the face of all that, it would be easy to dismiss the new breed of climate activists as noisy but dreamy, idealists distant from the powers who run the show. But that’s the thing about social movements, from civil rights to gay rights. At first the activists may look naive, and maybe they are. But in the rearview mirror, they come to look like instigators of massive and, in hindsight, obvious change.

–With reporting by Charlotte ALTER/WASHINGTON

This appears in the April 01, 2019 issue of TIME.

Write to Justin Worland at justin.worland@time.com.