I’m a Green. I’ve twice been the Australian Greens’ candidate for a seat in Australia’s federal parliament. But on Nov. 8, all of the good that the Green political movement has done since it was founded could be outweighed by the Green Party in the United States if Jill Stein, its candidate for president, brings about the election of Donald Trump.

We’ve been here before. In 2000, Al Gore would have become president if he had won Florida. George W. Bush won the state by 537 votes, while 97,241 Floridians voted for Ralph Nader, the Green candidate. Nader subsequently wrote on his website: “In the year 2000, exit polls reported that 25 percent of my voters would have voted for Bush, 38 percent would have voted for Gore and the rest would not have voted at all.” If we divide up Nader’s vote in that way, the result is that, without him in the race, Gore would have won Florida by more than 12,000 votes.

Before the election, a group of former activists for Nader published an open letter calling on him to end his campaign. “It is now clear,” they wrote, “that you might well give the White House to Bush.” Nader refused, saying that there was no significant difference between the two major party candidates.

We now know how wrong that was. If there had been no Nader candidacy in Florida, the U.S. would have elected the strongest advocate of urgent action on climate change ever to have held the presidency. Already in 1992, in his book “Earth in the Balance,” Gore had argued for such an agenda.

As America’s vice president, moreover, Gore signed, on behalf of President Bill Clinton’s administration, the Kyoto Protocol, the first serious international effort to limit greenhouse-gas emissions. Bush, in contrast, rejected the scientific research on climate change, withdrew the country’s signature from the Kyoto Protocol and for eight years effectively torpedoed international efforts to solve the problem.

Bush also made many other disastrous decisions, of course, the foremost being the unprovoked and unnecessary invasion of Iraq. The world is still struggling with the consequences of the destabilization of that region.

After that, you might have thought, no one would try the “no difference” line again -- and surely not in an election between Donald Trump and Hillary Clinton! After all, Clinton would not only be the first female U.S. president; she is also a long-standing campaigner for women, for health insurance and for gun control. In accepting the Democratic Party’s nomination, she said: “I believe in science, I believe that climate change is real.”

In contrast, Trump tweeted: “The concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive.” He later called that statement a joke; but he has also said -- not in a tweet, but in a major speech about economic policy -- that he would “cancel the Paris climate agreement” and “stop all payments of U.S. tax dollars to U.N. global warming programs.”

Yet, incredibly, Stein is sounding just like Nader in 2000. Asked if the prospect of a Trump presidency is equal to that of a Clinton one, she replied, “They both lead to the same place.” She then said that the Democrats “have better spin ... but they’re catastrophic as well.” In support of that, she added, “Just look at the policies under Obama on climate change.”

I’m looking. And the policies on climate change under Obama are vastly better than those under Bush. Obama’s policies made possible the Paris climate agreement concluded last December -- not enough, to be sure, but far better than anything Trump is likely to do. Given the Republican majority in the U.S. Congress, Obama has done well.

Will history repeat itself? I suspect that Trump would be an even worse president than George W. Bush, so I hope not. But Stein is on the ballot in Florida and Ohio, two big states that could decide the election. One recent poll gives her 3 percent of the vote, enough to make the difference in either of those states.

I call on Green party leaders all over the world to ask Stein to take her name off the ballot in states where the contest is likely to be close. If she won’t do it, they should take their appeal to voters, and ask them, in this election only, not to vote Green. The stakes are too high.

I understand the importance of changing the two-party system. What the U.S. needs to achieve this is voting reform. Greens should not be working to elect a Green president, which is impossible under the current system, but to institute a fairer voting system, perhaps like Australia’s, which uses what is known in the U.S. as an “instant runoff.” Voters rank the candidates in order of preference, and if no candidate receives a majority of all votes cast, the candidate with the fewest votes is eliminated. The votes received by that candidate would then be transferred in accordance with the second preferences of those who voted for him or her.

In the current election, Stein’s voters would be able to vote for their candidate without worrying that their choice might benefit Trump, who presumably would not be these voters’ second preference. If the U.S. had such a system, I wouldn’t be writing this column.

By Peter Singer

Peter Singer is professor of bioethics at Princeton University and laureate professor at the University of Melbourne. His books include “Animal Liberation,” “The Greens” (with Bob Brown), “One World” and “The Most Good You Can Do.” -- Ed. 

(Project Syndicate)