Godwin's Law, a principle devised in 1990 by lawyer and author Mike Godwin, goes like this: "As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one."

Similarly, when political debates are at their most extreme, the odds of some sort of invocation of Germany's 1930s and '40s narrow very considerably.

And thus, in Question Time yesterday, did we witness an extensive debate about whether the term "denier" was unparliamentary.

Christopher Pyne, the manager of Opposition business, demanded the Prime Minister withdraw her description of Tony Abbott as a "climate change denier".

"We all know the connotation that the Prime Minister is trying to bring about by using the word 'denier'," Mr Pyne argued.

"We know that she is trying to allude to the Holocaust. It is offensive, and it must stop."

Three weeks ago, Parliament House disappeared up its own fundament for half a day arguing about whether it was offensive for Sophie Mirabella to liken the Prime Minister's electoral deafness to that of Colonel Gaddafi.

Back then it was the Government taking offence; this time, it's the Coalition.

The prosaic Climate Change Minister, Greg Combet, wasn't having any of it yesterday.

"When you stop denying climate change, we'll stop calling you a denier. That's the fact of the matter," he told the Opposition, flatly.

It's a twisty one, because in point of fact the Leader of the Opposition denies being a denier anyway. He has told his footsoldiers the Coalition does not take issue with the science of climate change; only with the Government's chosen means of addressing it.

This is a debate about tax he impressed upon them at a party meeting several weeks back and not about science.

And most of Mr Abbott's colleagues, much as their opinions on this issue diverge, have maintained this stance (apart from the Western Australian MP Dennis Jensen, who on Monday cracked open a can of fizzy-pop and - indicating the bubbles - declared the escaping gas would be tax-liable under a Gillard carbon-pricing regime. Mr Jensen is very much his own man).

Julia Gillard, too, doesn't want this to be a moral debate.

She wants it to be about spanking-new green energy sources, and a national economy that hums along with purpose and can show its face at the G20 without embarrassment.

So how did we get to the Holocaust?

There's no doubt there were moral-crusade elements to the Rudd government's sale job on its emissions trading scheme; the powerful implicit, and sometimes explicit, suggestion that anyone harbouring doubts was guilty not only of foolishness but also of moral turpitude.

But does the use of the term "denier" knowingly invoke the Holocaust?

Or is such an interpretation just as inflammatory as objectors argue the term itself to be?

"Christopher Pyne is a very sincere and well-intentioned supporter of Jewish causes," says Jewish Labor MP Michael Danby.

"But it's sometimes a danger, whether you're Jewish or not Jewish, using everything to do with that issue when you're involved in any debate. It works in reverse, too; you can't use your sincerely-held views on that issue to authenticate your views on other issues.

"How can you deny the use of the word 'deny' in its wider parlance?"

Josh Frydenberg, the Jewish Liberal MP, told The Drum last night that in his opinion "Tony and Chris dealt with (the issue) appropriately."

Annabel Crabb is ABC Online's chief political writer.