Right now, no one knows your chosen third-party candidate exists. Let’s suppose you, personally, set out to change that. Let’s suppose you’re the best campaign manager ever, and that you and your friends lead the best grassroots campaign in history, so much so that come November you’ve convinced everyone who would have voted third party and two-thirds of the people who would have voted for Biden to vote for your candidate instead.
Let’s suppose that, had you done nothing, the vote breakdown will be the same in 2024 as it was in 2020. Biden got 51.3% of the popular vote, Trump got 46.8%, and that leaves just under 2% who voted third party. In 2024, thanks to your heroic efforts, the green party candidate gets 36.2% of the vote and Biden gets 17.1%, but oh no – Trump wins by a landslide because Trump supporters, who care not for your left-of-center politics, stuck by their guy and gave him 46% of the vote again, and in first-past-the-post voting, that’s all that matters! Your heroic efforts to get your message out about your better candidate only resulted in splitting the vote and ensuring Trump would win!