Something weird is happening in American politics: People are excited about climate policy again.

Since November, progressives have rallied around a Green New Deal, a package of policies aiming to slash carbon emissions while renewing the U.S. manufacturing sector. Though its finer details remain hazy, think tanks have begun to explore how it might work, and Democrats with White House ambitions have rushed to endorse it.

At least a decade has passed since climate change commanded so much political attention in the party. “If your 2020 platform doesn’t include a Green New Deal, are you really running for president?” teased Representative Alexandria Ocasio-Cortez a few weeks ago.

Yet for every imaginative proposal or boisterous protest, there is an unavoidable truth: Passing a Green New Deal is going to be really, really, really hard.

The reason for this difficulty is so simple and straightforward, it feels almost silly to mention. Success will require Democrats to control the White House, the House of Representatives, and the Senate—and then find a policy that will pass all three.

Read: The Green New Deal hits its first major snag

That is, of course, how you pass a law. But Democrats could very well find legislating a Green New Deal to be more arduous than achieving other progressive goals, such as Medicare for all or a wealth tax on super-millionaires. Not once, during the modern era of climate politics, has a major emissions-cutting bill made it through this gauntlet. And a Green New Deal has an urgency distinct from other party aspirations: The sooner it’s in place, the more time it will have for the marathon work of attacking emissions.

Thirty-one years have elapsed since the NASA scientist James Hansen told Congress that global warming was under way. Democrats controlled both the White House and Congress during four of those years, and twice—in 1993 and 2010—passed an ambitious climate-related bill out of the House only to see it die in the Senate. “The Senate,” concluded the writer David Roberts, “is where dreams go to die.”

In the next six years, immutable political facts will make passing anything out of the Senate especially difficult. Yet the next six years are the period that Green New Deal advocates must care most about. They have explicitly adopted the goal of limiting Earth’s temperature rise to 1.5 degrees Celsius, which would require global carbon emissions to be cut almost in half by 2030. That already appears next to impossible. It will be risible fantasy if no law is on the books by 2025, if not much earlier.

Below, I’ve described one path that Democrats must walk to turn any ambitious climate policy into law. It will require the party to make an unlikely journey. It might require mainstream Democrats to endorse far more aggressive governance than they would have ever once considered. But if the past few years have shown anything, it’s that the unlikely, the aggressive, and the unprecedented can come to pass.

What must Democrats do to pass a Green New Deal in 2020?

1. A Democrat must win the White House in 2020.

If Republicans retain control of the executive branch, the next opportunity to pass a Green New Deal would not come until after the 2024 election. It could still do much good then, but the window for policy to prevent 1.5 degrees Celsius of warming would have likely closed.

2. Assuming a Democrat is president, Democrats must retain control of the House of Representatives in 2020.

3. Democrats must also win the Senate in 2020. Because the vice president is a Democrat in our thought experiment, the party will need to win at least 50 seats to control the 100-member upper chamber. (The vice president can vote in the Senate in the event of a tie.) Thirty-five senators who caucus with the Democrats are not up for election in 2020. To control the Senate, the party must win at least 15 seats.

Where do those seats come from? Democrats are considered a lock to win seven states in deep-blue territory. Four more races in bluish states (Virginia, Minnesota, Michigan, and New Hampshire) will feature popular Democratic incumbents. And figure that Democrats win Colorado, which broke for Hillary in 2016.

That’s still only 12 seats, and Democrats need at least three more. So they will likely try to capture at least three of the following eight states: Arizona (where a Democrat won in 2018), Alabama (where the incumbent Democrat Doug Jones is running), Maine (against the popular GOP senator Susan Collins), Georgia, North Carolina, Iowa, Texas, and Montana. With the exception of Maine, Trump won all those states in 2016 with at least a 3.5-percent margin.

Yet those eight states are crucial only to winning back the Senate, not the presidency. If Democrats regain their usual footing in the Great Lakes—as they already did in the 2018 midterms—then they could still win 270 electoral votes without making a dent in the Senate. But then hope for any legislative Green New Deal would be dashed: The GOP Senate majority leader could simply decline to bring any Democratic proposal to the floor.

4. Let’s say Democrats pull it off—but just barely. By January 2021, a Democratic president is addressing a joint session of a Democratic House and a meager, 51-person Democratic Senate majority. Now it is, at least, possible to pass a Green New Deal. But party leaders must be willing to play hardball.

Under current Senate rules, most federal legislation must receive at least 60 votes to pass out of the chamber. This is not due to some constitutional requirement—any simple majority can pass a bill—but due to the filibuster, which requires a 60-vote “supermajority” to end debate on a measure. Because the filibuster is enshrined only in the Senate rules, not in the Constitution, Democrats could repeal it with 51 votes. The Senate has already repealed the filibuster when voting on Cabinet nominees and federal judges, but has so far preserved it when considering new legislation.

Read: What to do before the Green New Deal

In our thought experiment, the Senate will not have 60 Democrats, period. And party leadership will struggle to find nine Republicans who will vote to end debate on a Green New Deal package. So if Democrats want to pass it, they will likely have to choose to end the legislative filibuster.

But will Democrats have the stomach for doing so? It’s not clear. “If you don’t have 60 votes, it just means you haven’t done enough advocacy,” the senator and Democratic primary candidate Kirsten Gillibrand said on a recent episode of the podcast Pod Save America. (She also refused to say that she would push for its repeal as president, though it’s quite possible she was noncommittal only so she could ding Republicans if they repeal the rule first.)

5. Democrats do have one way to try to work around the filibuster. They might try to pass a Green New Deal through “budget reconciliation,” an odd loophole that allows roughly one bill per fiscal year to pass out of the Senate with a simple majority of votes. But reconciliation can only be used to pass a bill that directly pertains to the federal budget—and it cannot be used to do anything that would significantly increase the federal deficit in 10 years’ time. So the most ambitious Green New Deal ideas would likely be ruled too broad to qualify for reconciliation. (Though the policy scholar Mike Konczal has suggested that passing Medicare for all in reconciliation could free federal funds for climate policy.)

6. But suppose Democrats ditch the filibuster. Then they must advance a Green New Deal bill that wins the support of every Democratic member (assuming that they only hold 50 seats). The party’s caucus will likely include several newly elected moderate Democrats—and it will almost certainly include Senator Joe Manchin of West Virginia. Manchin, who has deep ties to the coal industry, has not been a friend to climate policy. When he first ran for Senate in 2010, he cut a campaign ad where he shot a hole through President Barack Obama’s favored climate bill. Yet he would need to support any party-line vote with a 51-member Democratic caucus.

7. Winning over Manchin explicitly for a Green New Deal is not the party’s only option: Democrats could change their own math in the Senate. Assuming they repeal the filibuster, Democrats would only need 51 votes to add a new state to the union. Luckily for them, two Democratic-leaning states would love to join: the District of Columbia and Puerto Rico. If Congress and the White House welcomed both D.C. and P.R. to the Union, Democrats could immediately pad their majority with as many as four new senators. And those senators could even be seated immediately. But this plan would also require the support of Manchin and any new moderates like him.

Or maybe every contingency here will fall through in 2021. Say that Democrats choose not to ditch the filibuster, not to use reconciliation, and not to add D.C. and Puerto Rico as states. Instead, they look ahead to the 2022 Senate election, possibly the most favorable cycle for Democrats in years. It offers the chance to seize some purple seats that Republicans won in 2016, with elections in Florida, Pennsylvania, Wisconsin, Georgia, North Carolina, and Ohio. Democrats finally enhance their majority.

Yet Democrats might come into their new Senate majority exactly as it becomes legislatively useless. Assuming a Democrat holds the White House, history suggests that the party could stand to lose dozens of House seats in 2022—and possibly control of the chamber, too. As Gallup wrote before the 2018 election: “The president’s party almost always suffers a net loss of U.S. House seats in midterm elections.”

With that loss, the Green New Deal will meet the same fate as Democratic climate policies before it. It will fail. The United States will neither cut its emissions via policy nor pour hundreds of millions into clean-energy R&D. And the planet’s temperature will keep soaring, well past 2 degrees Celsius.

Or maybe not—I’ve only given one scenario here, after all, and reality could differ in countless ways. Every additional Senate seat that Democrats take in 2020 could help them pass a Green New Deal. Sudden demographic change—such as a surge in enthusiasm among young voters—could push electoral math in Democrats’ direction. And if a Green New Deal fails, the party could also make a “second go” at climate policy before the 2022 election—trying for something like the carbon-fee-and-dividend scheme endorsed by many Democrats, some Republicans, and most oil companies.

The technological outlook could also shift. In 2007, few predicted that the sudden success of fracking would alter the next president’s energy policy. Yet it did. A breakthrough on a technology like carbon capture—which is financially supported by many oil firms—could rapidly scramble the partisan politics of climate change, making the GOP more willing to negotiate on a round of federal spending.

Yet none of these possibilities changes the basic fact that a Green New Deal will be really, really, really hard to pass in the coming years. And if we want to avert disastrous climate change, the coming years are the most important ones we have.