While the letter thanks Stein, the Green Party candidate, and West, who is running as an Independent, "for raising important issues in this election," the groups argued that Harris "is the only candidate with a record of success addressing climate change," pointing to her tie-breaking vote in support of the Inflation Reduction Act and legal action against oil companies during her tenure as California's attorney general.
Trump, by contrast, "waged the worst White House attack ever against the environment and public health while in office," the groups wrote.
One analysis estimates that a second Trump presidency could result in an additional 4 billion tonnes of carbon dioxide equivalent emissions by 2030, negating recent progress in renewable energy development and inflicting large-scale climate damage. Fossil fuel industry attorneys and lobbyists are already drawing up executive orders for Trump to sign should he prevail on November 5.
"He has made clear that his second term will be even more extreme, drawing from the detailed anti-environment proposals and plans contained in Project 2025," the climate coalition wrote in the open letter. "Every vote for Dr. Jill Stein or Cornel West instead of Kamala Harris makes it more likely that Donald Trump will win."
In a separate social media thread on Thursday, the youth-led Sunrise Movement—a signatory of the new open letter—wrote that "this election will decide the temperature on our planet for thousands of years."
"It will decide if we have a fighting chance to stop the climate crisis or not," the group added. "Voting alone or voting third party won't save us. Let's be honest—financing a genocide, competing on who can be crueler to immigrants, or thumbs-upping fracking is fucked. A Harris presidency won't stop violence. No president ever will."
"But neither will disengaging or throwing away our votes. This election might not save the world but it will set the scene," said Sunrise, which has been working to mobilize young voters in swing states to back Harris. "We have six years to stop the climate crisis and we can't afford to give four of them away to Trump. What your summers look like in 2047, where your family might live in 2063, whether tens of millions of more people become climate refugees or not will be deeply impacted by the election results."
Tens of millions of ballots have already been cast in dozens of states across the U.S. ahead of Election Day, with officials reporting record turnout in battlegrounds such as Michigan and Georgia.
Stein and West are among several third-party candidates on the ballot in critical states that could decide the presidential contest. Recent polling has shown that Stein and West are both polling around 1% in the key state of Michigan, where Harris and Trump are in a dead heat.
Michigan has received significant attention this election cycle given that it was the birthplace of the Uncommitted National Movement, which began as a primary campaign effort to push President Joe Biden to halt U.S. support for Israel's assault on the Gaza Strip.
The movement has since shifted its focus to pressuring Harris to back an arms embargo against Israel, something she has declined to do. While Uncommitted opted against endorsing Harris last month, it said it opposes the Republican nominee and warned that "third-party votes in key swing states could help inadvertently deliver a Trump presidency given our country's broken Electoral College system."
"We are going to be voting for the world's climate future even if we are reluctant to admit it."
Stein has dismissed the notion that she's a potential "spoiler" candidate, arguing her supporters would likely opt to stay home instead of vote for Harris or Trump if there was no third-party choice.
But speaking in Michigan earlier this month, Stein supporter Kshama Sawant—a former member of the Seattle City Council—acknowledged that "we are not in a position to win the White House" and said she views the Green Party candidacy as an opportunity to "deny Kamala Harris the state of Michigan."
"And the polls show that most likely Harris cannot win the election without Michigan," Sawant added.
That approach could be devastating for the planet, progressive lawmakers and climate advocates have argued.
"If Donald Trump is elected, the struggle against climate change is over," Sen. Bernie Sanders (I-Vt.) wrote in a social media post on Thursday. "The United States will withdraw from the movement toward sustainable energy."
Kumar Venkat, a carbon footprint analyst, wrote in an op-ed for Common Dreams on Friday that "if elected, it is reasonable to expect that Harris will build on the Biden administration's work and keep the U.S. on the net-zero path."
"Donald Trump has made it exceedingly clear that he does not believe climate change is even a problem," Venkat wrote. "Between Project 2025 pushing for a 'whole-of-government unwinding' of U.S. climate policy and the fossil fuel industry drafting detailed plans to dismantle the Biden administration's climate rules, it is a safe bet that we will no longer be on a trajectory to net-zero emissions if Trump is back in the White House."
"We are going to be voting for the world's climate future," he added, "even if we are reluctant to admit it."