If society has evolved to a point that Hitler stands a good chance of being elected, it won't matter. If Hitler is able to command 49% popular support, the formality of an election is not going to be an obstacle that keeps him from office. An election is only relevant if people recognize its authority.
In Tennessee in the 1820s the Governor and Legislature were tired of the Tennessee Supreme Court striking down their laws. So they organized their own Supreme Court and started ignoring the old one. The old court continued meeting and issuing rulings but the Governor and Legislature pretended it didn't exist and started bringing cases to the new court. The fact the old court was the legally constituted court was irrelevant if everyone simply ignored it.
This is one of the few cases in which, if you ignore something, it actually does go away. If enough people ignore the government, the government eventually just ... stops. Even people who advocate for compulsory voting are very clear about the reasons they want it - "Voting confers legitimacy ... as more and more people choose to abstain, elected officials rule with less and less consent."
To be clear, I am not proposing that the current system is the best option. My point is that the "lesser of two evils" vote can be appropriate in the right circumstances. We have ways of knowing to some degree how many people are going to vote. It is then more or less a fact
that someone is going to be elected whether you like it or not. There may be a threshold where the number of non-voters requires a restructuring but if the upcoming election is nowhere near this point it seems irrational to not
vote for the lesser of the two evils. The best tactic to me seems to answer in polls that you aren't going to vote but if things get hairy, go and vote anyway against the guy you really
don't want to be prez. A continuance down this path would surely lead to less voting in the long run anyway.
You've disqualified the Hitler example on the grounds that if Hitler commanded 49% of the vote it would mean that the system was not respected as an authority but you are assuming the reasons for non-voting are other than apathy. If apathy were the reason for low voter turnout after all, why would the number of voters matter? As long as a system is in place and they are relatively comfortable, apathetic citizens don't care by definition. However, they may still fear punishment from the law. If we pare down the Hitler example to a very small voting population it could be actualized. Not Hitler himself obviously but some simulation. So in this theoretical example the voting population is small enough that Hitler is
getting those kinds of percentages. In this situation it would be in the best interest of those concerned to get off their asses and vote for the other guy. I don't really think the size of the voting population enters into the validity of the argument but I feel that the pared down example at least proves that the "lesser of two evils" vote can be validated.