When I was at university, I remember reading a book about nuclear weapons that had been written in the early 1980s. (It's quite a famous book, I think, but the name of it temporarily escapes me.) It started by pointing out that opinion poll evidence showed that roughly half of the UK's population expected that, at some point within their own lifespans, human civilisation would be destroyed by a nuclear exchange between NATO and the Warsaw Pact. It then pointed out that it was somewhat odd that it wasn't the number one political priority of both the electorate and the government to prevent that seemingly imminent catastrophe from occurring.
Arguably, we find ourselves in quite a similar situation now, because I genuinely think that if Donald Trump is elected US President in November, there is a non-trivial chance of a nuclear war occurring within his term of office. That's not so much because of his insane right-wing views (it's difficult to know how many of those are genuine and how many are poses for the benefit of a Republican audience), but more because he's an ultra-belligerent narcissist with very little sense of restraint. If he ever got the US into a serious dispute with Russia or China, it's not at all hard to see how it could escalate quickly. The other day, I speculated that the chances of nuclear war under President Trump were at least 2% or 3%. RevStu reckoned I should have stuck a zero at the end of those numbers.
Whichever of us is right, it's surely clear that the risk is unacceptably high. As there can be nothing more important than preventing our species from being destroyed or mostly destroyed, it therefore follows that absolutely every other political consideration must play second-fiddle to the need to stop Trump. The problem is that it's not at all clear how that is most likely to be achieved. Most polls actually show that Bernie Sanders would have a better chance of beating Trump in a straight fight than Hillary Clinton would. The cheerleaders for Clinton point out that the public at large haven't taken a close enough look at Sanders yet, and when it really comes down to it they would never elect a self-declared socialist. But Clinton has her own disadvantages - she's a hate-figure for many moderate conservatives in a way that Sanders isn't, so it's far from inconceivable that she could genuinely prove to be the more unelectable of the two.
In view of that uncertainty, I think all that Democratic primary voters can reasonably do is forget about Trump for the time being, and vote for the candidate they really believe in. For my own part, I had no hesitation in casting my postal vote for Sanders a few days ago. It's almost painful to think what a huge step forward it would be for the world if he was elected - America would come into line with global opinion on the Israeli-Palestinian conflict, nuclear weapons would be drastically reduced, and there would be a passionate advocate in the White House for the eradication of the death penalty, which could have huge knock-on benefits for human rights in China and the Middle East.
But if by any chance the eventual contest is Sanders v Trump, it'll be a straight all-or-nothing choice between America becoming a normal western country at last, and a descent into madness and possibly darkness. Without question it'll be the scariest election anywhere in the world since Germany went to the polls in 1932.