I used to think the scenario you presented of the conservatives voting against their interests was exclusively their issue. The example you gave was the one I would use.
But I have watched my progressive friends — as a progressive these are a lot of my friends — stubbornly vote third party in 2000 and this year out of some kind of liberal purity test. If you’re on the Left, and your guy loses, you do the self-interested thing and get the remaining Left wing candidate elected. You don’t burn the house down and let the conservatives get elected. But an ideological stubbornness keeps people repeating the tired saw about lesser evil candidates. As far as I am concerned, if you look at the totality of our history, this strategy of not supporting the Left wing candidate has moved the country further and further right.