There's a bellicose undertone in a lot of Leftist rhetoric these days. People who preach peace and tolerance, rainbows and unicorns, can switch to a very different wavelength the moment they start talking about conservatives.

Listen, for example, to the way even mainstream Democrats discuss Donald Trump's healthcare reforms.

Here is Hillary Clinton: "Forget death panels. If Republicans pass this bill, they're the death party."

Here is Elizabeth Warren: "I've read the Republican ‘healthcare' bill. This is blood money. They're paying for tax cuts with American lives".

And those are the leaders, for heaven's sake. Gaze into the maelstrom of left-wing Twitter and Facebook activism, and you find an altogether less restrained tone. Republicans, you read, are monsters who have gone into politics for the express purpose of murdering the poor. They are not political opponents; they are enemies of humanity, vermin.

Here is a typical example from a Bernie Sanders supporter, who saw the healthcare reforms as proof that Republicans hated the working class:

"Trump is a Traitor. Trump Has Destroyed Our Democracy. It's Time to Destroy Trump & Co."

Not long afterwards, the man who posted those words went to a congressional baseball practice session intent on shooting Republicans and left a GOP Rep. Steve Scalise in critical condition.

You can just about follow his logic. Even many moderate leftists now seem to agree that Republicans are murderers, intent on killing low-income Americans for some opaque reason of their own — possibly sheer sadism. And, if Republicans are murderers, then getting your shot in first is a form of self-defense, is it not?

For a brief moment, as happens in the aftermath of these atrocities, all sides came together and stressed the things they had in common and condemned violent rhetoric and yadda yadda. But, within less than a month, leading Dems were back to calling the GOP "the death party."

Don't get me wrong: The only person responsible for the Alexandria abomination was the perpetrator. Sure, it's hypocritical to complain about the rhetoric of the NRA or about using a crosshair as a metaphor for targeting a seat and then to talk about Republican "blood money." But hypocrisy is not the same thing as complicity.

Still, we should be clear about where the legitimation of political force can lead. In Britain, we have recently seen a sudden rise in violent protests, partly because a group of Trotskyists has taken over our Labor Party. The Labor leader, Jeremy Corbyn, refuses to accept the recent election result, possibly seeing Parliament as a bourgeois institution. His deputy, John McDonnell, called for a million people to take to the streets and "force Theresa May from office". A "Day of Rage" was duly scheduled (the terminology borrowed directly from Hamas), but happened to fall during a heatwave – and, as you may have noticed, Brits are never much use in the heat.

Just as in the United States, the language of dehumanization – what Leftists call "othering" when rightists do it – is becoming commonplace. Following a recent tragedy, in which a tower block burned down, McDonnell accused the Conservatives of murder, as though they had somehow started the fire. Unsurprisingly, there have been violent demonstrations since the blaze: Why wouldn't you hit back at politicians who – again, for vague and unspecified reasons – want to exterminate the poor?

You thought of Britain as a civil and courteous country? A country whose people say please and thank you, a country of orderly queues, a country whose police go unarmed? That's what we thought, too. See how quickly thuggish rhetoric can debase a political culture.

Let me spell it out. Either physical force is legitimate in democratic politics, or it isn't. If you allow it in any circumstances, then its exercise becomes a question of tactics, not principle.

If it's okay to stop Milo Yiannopoulos or Ann Coulter from speaking on campus by rioting, then political violence is legitimate. If political violence is legitimate, then the question of who is an acceptable target becomes one of personal choice. For James T. Hodgkinson, the Alexandria shooter, it was Republicans. For Dylann Roof, the Charleston shooter, it was black people. For Micah Johnson, the Dallas shooter, it was white people. It's all a matter of taste.

Every act of physical coercion – throwing paint over a politician, vandalizing the property of an arms manufacturer, preventing a speaker from reaching a podium – weakens the taboo against violence. The difference between "punch a Nazi" and "shoot a Republican" is merely one of degree.

Dan Hannan is a British Conservative MEP.