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.