All political parties frequently spend a lot of their time criticising their opponents, which is a legitimate thing to do but is often overdone, and even the most normally truthful of politicians seem to have to guard against a propensity to go "over the top" when talking about their rivals.
One of the many reasons why this is a bad idea is if you have grossly exaggerated the threat posed by your opponents in the past, it has a diminishing effect, and also leaves you with no good way to say "look guys, we really mean it this time."
For example, the Labour party has claimed in every election for the past fifty years or so that the Conservatives will destroy the NHS if they get into or stay in power. As the Conservatives have been in office after seven of those elections and 29 of the last 50 years, and the NHS is still here, just about everyone in the country who isn't hopelessly gullible or a total cretin has worked out that these allegations are at least an overstatement of the case. The majority of voters take them with a large pinch of salt.
What on earth would Labour say in the unlikely event that they were ever up against a party which really did want to destroy the NHS?
The Democrats had a problem with Donald Trump which was not dissimilar to this in 2016 and the Conservatives have more than a hint of the same issue when dealing with Jeremy Corbyn now.
Now I would not for a moment accept that the Conservative campaign against Ed Miliband misrepresented the truth of his position to anything like the same extent that Labour routinely misrepresents the Conservative position and record on the National Health Service.
But we do have to find a way to get over that the present leadership of the Labour party is genuinely different to anything we have seen in this country before. They stand for a much more extreme brand of a philosophy of which a mild version nearly wrecked this country half a century ago.
Ironically the Labour leadership themselves and their supporters in Momentum are keen to explain the same thing. Momentum keep telling some of their own MPs and councillors to "**** off and join the Tories" because compared to them people like Michael Foot and Neil Kinnock look like Liberal Democrats and a good chunk of their own party do look like Tories.
In the clip below from the 2016 US presidential campaign, Democrat supporter and comedian hit this issue head on: he's a comedian and what he says is meant to be funny but he's not really joking.
Paraphrasing wildly, Maher more or less apologises for most of the rude things that he and other Democrats had said about Mitt Romney, John McCain and even George W Bush, calling them "honourable men who we disagreed with and we should have left it that way" and essentially saying that liberals had been crying wolf in their attacks on previous Republican candidates but that Donald J Trump really was everything they had said about the previous candidates.
It didn't work. I can understand why.
"We were wrong before but we're telling the truth now" is not the most persuasive of slogans.
But I cannot help thinking that he was onto something in the belief which prompted him to make the speech in the clip below, that the criticisms which he and others had made about previous republican candidates were much more true of Trump.
A similar principle applies to the Corbyn leadership of the Labour party, which is why Dan Hannan's piece above felt to me as though he was channelling Bill Maher.
In terms of their wish to dismantle the existing order and in a number of other respects Trump has far more in common with Corbyn that either would like to think.