It's a tricky example because the Republican was effectively the third party candidate in that election. But once you realize that, the spoiler effect is very e

It's a tricky example because the Republican was effectively the third party candidate in that election. But once you realize that, the spoiler effect is very easy to see; the Republicans would have mostly preferred the more centrist Democrat candidate over the progressive Independent candidate, and if they had strategically ranked the Democrat first on their ballots the Democrat would have won. By voting honestly and putting their favorite candidate first, they screwed themselves over and threw away their vote, and were understandably bitter about it given that ranked choice is frequently claimed to solve this exact problem.