In the end I voted for Harris, and I did the deed weeks ago, so I've had plenty of time to mull it over, and it does make me feel dirty. I'm pretty sure the me of fifteen years ago would have taken the opposite decision, and the me of fifteen years ago might well have been right. If I'd heard Bill Clinton's astounding pro-genocide speech before rather than after I voted, that might possibly have changed my decision, I don't know. But for what it's worth, my reasoning was as follows -
* I just couldn't see what a vote for Stein was actually going to achieve. If she had a realistic chance of getting to 5% of the vote, that would have swayed me, because it would have unlocked federal funding for the Greens. Even a realistic chance of getting to 3% would have interested me, because that might have started a serious conversation among Democrats about the policy changes required to win that substantial bloc of votes back. But at 1% or less of the vote for Stein, even if that's enough to swing the election in Trump's favour, the Democrats will stay in their comfort zone and the only lesson they'll learn is that they need to lecture voters even more about not wasting their votes.
* It's quite true that the first-past-the-post voting system forces you to choose at times between a glorified abstention and casting what is effectively a fifth-preference vote to prevent your sixth preference from winning. In a sensible America with a preferential voting system, I wouldn't have felt remotely conflicted about giving Harris a fifth preference vote to make sure I ranked her above Trump, and arguably the principle of voting for her tactically under FPTP is much the same.
* Harris is only one of two people on the Democratic ticket, and I gather Tim Walz is regarded as having had a left-ish record as governor of Minnesota (although he's no better than Harris on the genocide issue). So that at least sprinkles a little glitter on the Democratic option.
The bottom line is that I always knew I would regret my decision regardless of which way I jumped, because there was no good option available. I'm certainly not going to be willing Harris to win tomorrow night, but at least it won't be on my conscience if she doesn't.
If it wasn't for the fact that Trump is certain to be irresponsible on climate change, and is not the sort of person you'd ever want to put in total control of the world's second-largest nuclear arsenal, there would be an argument that a Trump win might even be the least worst option, because it would undermine American leadership of the 'international community', which has been so utterly toxic for decades. For example, why else are some European countries so slavishly loyal to the Netanyahu regime, no matter how many atrocities it commits? It's because US politicians are bought and paid for by the Israel lobby, and many European governments think, say and do whatever the Americans tell them to think, say and do. A Trump win could indirectly stop that destructive cycle, because European voters will no longer recognise the US government as representing moral leadership.