Just a general observation. Trump and Hitler get a mention and all the talk is about the right making a comeback, oops I mean the far right.
As far as the OP goes, what's new? Show me a political party and I will show you false promises.
I don't, as I made clear in the OP, think there was a golden age when this didn't happen. I also don't think that's the point the article is making. Though I think the article gets the target wrong.
We can certainly argue that there has been a coarsening of the political discourses, and you often see it done when people show detailed discussions being carried out historically on TV, or pull out the speeches from the Lincoln - Douglas presidential debates. I think that views the past through the rosy coloured views of cherry picking. However, I think politics is becoming at least in sound more polarised, and I don't mean by that between left and right. Rather I think that that even small divisions become exaggerated, due to the ease which the loudest voices get heard.
To take your point, you are completely correct that there is a lot of this type of rhetoric from the 'left'. The corrosive attitude that everyone who voted for Brexit was a racist, or that anyone voting for Trump was a redneck loon is just another example of the 'certainty' the article refers to. And Jack Knave is right here in noting that as ever 'the public gets what the public wants', perhaps even more so given the echo chambers of the net, and news media.
Further as wigginhall has raised, in times of insecurity we seek certainty, but we are almost bound to live in the interesting times, Rose mentioned. The sheer speed of change in technology, change in job patterns, change in social patterns is extraordinary and getting faster. Added to that, the ability both to see it happening in the 24 hour news cycle, the ability to then talk to others immediately, and the difficulty in having time to investigate anything to get any reasonable certainty as to truth, and we seem duty bound to grasp for a lifejacket of certainty.
To take a trivial example, the stushie in the last few days over the comments on under age *s*e*x by Milo Yiannopolous, something that will pass many people by, have generated huge amounts of writing, and arguing, and personal attacks, across the politisphere. And in many of that this 'conversation' I have seen people entrench themselves deeper and deeper into a Milo is a martyr for free speech or he's a disgusting supporter of child abuse. And all through it, due the nature of the tapes that were released, the trolling nature of some of his previous comments, and the deafening shouting of views from all points on the web, I don't see how people could take an informed opinion without spending huge amounts of time investigating what is an unimportant case.
I am at a loss how we control it, and indeed am conscious that despite my best efforts, I have contributed to it. I am, as ever, haunted by the words of Yeats that 'the best lack all conviction, while the worst are full of passionate intensity' - words which are a clearer summing up Massie's article, and indicate that this isn't a new issue. I think though that Yeats has it wrong, and that what the best and the worst of us need to do is question how we debate and contribute to the discourse. To avoid the easy traps of generalizations, the constant othering, to admit that now more than ever we can be wrong.