Events have a multiplier effect. And when they come in bunches the effect can be overpowering. This was already a sad and demeaning day, even before we heard the ghastly news a Labour MP, Jo Cox, had been murdered outside her constituency surgery in Yorkshire.

Politics is, figuratively speaking, a contact sport. It is a hard business because it is an important business. It matters and it matters even more when the stakes are so very high. But just as class will out at the highest level in sport, when the stakes are the very greatest and everything seems to be on the line, so character reveals itself in politics too. Even, especially, when it really counts.

A referendum is one of those moments when it counts. There is no do-over, no consoling thought in defeat that, at least, there's always next season. No, defeat is permanent and for keeps. That's why a referendum is so much uglier than a general election. The 'wrong' people often win an election but their victory is only - and always - temporary. There will be another day, another time. An election is a negotiation; a referendum is a judgement with no court of appeal. So character reveals itself. The poster unveiled by Nigel Farage this morning marked a new low, even for him.

The mask - the pawky, gin o'clock, you know what I mean, mask - didn't slip because there was no mask at all. BREAKING POINT, it screamed above a queue of dusky-hued refugees waiting to cross a border. The message was not very subtle: Vote Leave, Britain, or be over-run by brown people. Take control. Take back our country. You know what I mean, don't you: If you want a Turk - or a Syrian - for a neighbour, vote Remain. Simple. Common sense. Innit?

And then this afternoon, a 42 year old member of parliament, who happens - and this may prove to have been more than a coincidence - to have been an MP who lobbied for Britain to do more to assist the desperate people fleeing Syria's charnel house, was shot and stabbed and murdered.

Events have a multiplier effect.

It may be that eyewitness reports he shouted ‘put Britain First’ as he attacked Jo Cox will prove as unreliable as such reports often are. It could be there was no political motivation for this apparently senseless murder. He has been named locally as Tommy Mair: his younger brother, Scott, had this to say:-

“ “I am struggling to believe what has happened. My brother is not violent and is not all that political. I don’t even know who he votes for. He has a history of mental illness, but he has had help.

We wouldn't have to ask quite so many awkward questions if this proves to be just - a relative term - another deranged act perpetrated by a suspect with a long history of mental illness.

But we know that even lone lunatics don't live in a bubble. They are influenced by outside events. That's why, when there is an act of Islamist terrorism, we quite rightly want to know if it was, implicitly or explicitly, encouraged by other actors. We do not believe - at least we should not - in collective guilt or punishment but we do want to know, with reason, whether an individual assassin was inspired by ideology or religion or hate-speech or any of a hundred other possible motivating factors. We do not hold all muslims accountable for the violence carried out in the name of their prophet but nor can we avoid the ugly, unpalatable, truth that, as far as the perpetrator is concerned, he (it is almost always he) is acting in the service of his view of his religion. He has a cause, no matter how warped it may be. And so we ask who influenced him? We ask, how did it come to this?

So, no, Nigel Farage isn't responsible for Jo Cox's murder. And nor is the Leave campaign. But they are responsible for the manner in which they have pressed their argument. They weren't to know something like this was going to happen, of course, and they will be just as shocked and horrified by it as anyone else.

But, still. Look. When you encourage rage you cannot then feign surprise when people become enraged. You cannot turn around and say, 'Mate, you weren't supposed to take it so seriously. It's just a game, just a ploy, a strategy for winning votes.'

When you shout BREAKING POINT over and over again, you don't get to be surprised when someone breaks. When you present politics as a matter of life and death, as a question of national survival, don't be surprised if someone takes you at your word. You didn't make them do it, no, but you didn't do much to stop it either.

Sometimes rhetoric has consequences. If you spend days, weeks, months, years telling people they are under threat, that their country has been stolen from them, that they have been betrayed and sold down the river, that their birthright has been pilfered, that their problem is they're too slow to realise any of this is happening, that their problem is they're not sufficiently mad as hell, then at some point, in some place, something or someone is going to snap. And then something terrible is going to happen.

We can't control the weather but, in politics, we can control the climate in which the weather happens. That's on us, all of us, whatever side of any given argument we happen to be. Today, it feels like we've done something terrible to that climate.

Sad doesn't begin to cover it. This is worse, much worse, than just sad. This is a day of infamy, a day in which we should all feel angry and ashamed. Because if you don't feel a little ashamed - if you don't feel sick, right now, wherever you are reading this - then something's gone wrong with you somewhere.

Jo Cox was, by all accounts, a fine parliamentarian and a fine woman. She has been taken from her family and her constituents but her death strips something from all of us as well. I cannot recall ever feeling worse about this country and its politics than is the case right now.

Events have a multiplier effect. So do feelings.

This blog has been updated to incorporate recent developments