It should be a civic duty but it is not, otherwise everyone would already have been automatically registered without exceptions, there would be an election day, and there would be less money involved. We are trying to get there, but currently this is not the case and the people you are claiming are not doing their duty still are. Duty do not mean "pick the candidate that will win", it is "pick the candidate that best defines you and the goal you want for the country." This is regardless of the candidate's chance of winning, that is what the party is for, to establish candidates that can win majority of the people's vote through their policies.
I don't know why you fear his election, only way he wins is if a dramatic amount of democrats do not show up for voting and this is the party's fault more-so than the voters.
Listen all that this is about is respect one another's choice, so long as it isn't an extreme choice *cough*Trumpvoter*cough* that completely disregards all the facts. This is about U.S election, it is very likely you won't change their mind by going about being a condescending prick basically, unless that isn't even your goal.
I'm going to make this clear, once again, and listen to it bluntly. I will not respect people who choose not to vote and sit at home while someone is running under a fascist and bigoted platform, nor will I respect them for voting foolishly on a 3rd party under a naive thinking of "protest".
I have plenty to fear, my life is always in the hand of the majority because I'm already a heavily oppressed minority (trans), to see people squabble and screaming "Bernie or Bust" over some of the most unintelligent, petty, and downright bizarre conspiracy theories that have been proven time and time again to be false is terrifying and anger inducing.
And ultimately, I'm tired of the mentality of "lol, I vote who I want", like this is a fucking game. I'm not entertaining these mentalities anymore after I've seen my own mother cry in fear of another Salazar type being in power, something she and the rest of my family from her side felt hard, nor will I entertain it after seeing how Trump's rise of power has made bigotry out of the closet and the norm to a lot of people.
I also hate the mentality that a vote should only go to what you think is best, what best represents you. It's the most selfish mentality one can have. You should vote for what will help everyone, what is best for everyone. You have that power, but you decided to use it for selfish wants and needs. I do not have that luxury, nor will I ever if this current climate continues, it's something I can't risk when the lives of myself and so many others that I have met, worked with, and befriended are on the line.
I'm not going to be a condescending prick, but I want you and others to know the true reality of things, of how this effects everyone else that maybe you'll never meet. What you do is your choice, I won't call you anything nor will I hate you, but I will not respect you if you sit in home and do nothing, or vote 3rd party under "protest".