Jill Stein, the Green Party's candidate did well in the 2016 Presidential Election, from Wiki: "Stein finished in 4th with over 1,457,216 votes (more than the previous three Green tickets combined) and 1.07% of the popular vote".
In the 2020 election, the Green Party candidate only got 339,000 votes. An apparent failure, but actually nothing of the sort. The Democrat strategists knew that losing votes to the Green candidate probably cost Hillary Clinton the 2016 election, so their 2020 candidate Joe Biden said he was in a favour of a Green New Deal (whatever that is) and clawed most of those votes back. Given how tight the margins were in swing states, that was a very sensible tactic. This is called "shifting the Overton Window", and now the Greens just have to hope he actually implements it.
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A similar thing happened to the Libertarian Party, their vote share went down from a respectable 4.5 milion to 1.5 million votes (as far as I can make out), presumably because Trump (the very antithesis of actual libertarianism) took back most of the votes from right wing nutters who otherwise might have voted Libertarian.
But fair play to the 2020 Libertarian candidate Jo Jorgenson, she is realistic about all this and understand how it works. From the BBC:
"The Libertarian Party's baseline votes will continue to grow [sic]," Ms Jorgensen said in a statement. "The only way Democrats and Republicans can keep us down is by adopting our libertarian policies."
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To cut a long story short, the lesson for small parties is - if your policies and principles become widely acceptable and acceptable, the larger parties adopt them (or at least pretend to) and you lose votes.
Debate Magazine
