Contrary to popular belief, it wasn't Hitler's propaganda minister, Joseph Goebbels, who said: "If you tell a lie big enough and keep repeating it, people will eventually come to believe it."
It might have been Lenin, who is often quoted as saying (although I can find no reliable source): "A lie told often enough becomes the truth." Who would have thought that, deep down, when he's not being all pumped up with new-found passion, David Cameron might well be a secret Leninist?
His claim, repeated ad absurdum, that a minority Labour government would be held hostage by the SNP is, I'm afraid, nothing less than a Great Big Lie. The truth is that the Lie could only become Truth if the Tories wish it so. And that is not something that Mr Cameron will ever admit to you.
(He was at it again during last night's BBC Question Time programme, during which, out of nowhere, he suddenly referred to Ed Miliband "potentially propped up by the SNP.")
Yes, I know, shock, horror, politicians tell lies during election campaigns. Lies like they'll increase spending, reduce borrowing, and not put up any taxes, while simultaneously equipping pigs with magic wings to enable them to fly over every polling station in the land next Thursday.
But back to Labour and the SNP. The Tories would have you believe that a minority Miliband government would be forced to abandon its fiscal responsibility programme, scrap Trident, and rain goodies galore upon the heads of Scottish voters, all in return for SNP votes in the House of Commons.
The SNP are perfectly happy for you to believe this nonsense as well - because it enables them to say to Scottish Labour voters: "Don't worry, you can switch your vote to us to give those pesky southerners a bloody nose, but you'll still get a Labour government, and we'll make sure it's the sort of Labour government you want, with no more of that wishy-washy Blairite stuff we had to put up with before."
It. Is. A. Lie. And I can demonstrate why by asking you to consider the following two scenarios.
Scenario A: A minority Labour government proposes a budget that includes cuts to public spending that the SNP don't like. They threaten to vote against the finance bill unless Miliband backs down. What do the Tories do? Vote against a budget that includes precisely the sort of spending cuts they would have introduced, or vote for them, with Labour, and neutralise the SNP threat?
Scenario B: A minority Labour government proposes a package of defence spending that includes a commitment to a like-for-like Trident replacement programme. The SNP are committed to vote against such a proposal and intend to do so. What do the Tories do? Vote against a bill that they thoroughly approve of, or vote for it and neutralise the SNP threat?
It's obvious enough, isn't it? On each and every occasion that the SNP try to threaten to bring down a minority Labour government, it will be within the Tories' power to prevent them. They don't even have to vote with Labour if that sticks too much in their gullet - abstentions will suffice. That's why I believe that the SNP will not in reality be able to hold a minority Labour government hostage unless the Tories let them. If you were feeling mischievous, you could call it a de facto grand coalition in all but name.
The only reason why the Tories might want to help the SNP is if they believed they could muster a Commons majority instead of Labour. But if that had been the case, they would already have done so, since Mr Cameron, as the incumbent next Friday morning, will get first crack at forming a new administration.
There is also, of course, the small matter of the SNP leader Nicola Sturgeon having committed her party to kicking the Tories out of Downing Street. It might be a tad tricky for her to explain to her supporters a Commons vote which could, in theory, usher the Tories back in again. That's why, somewhat earlier than most commentators, I was already suggesting two weeks ago that Miliband had called the SNP's bluff.
It's important, by the way, to look extremely carefully at the formula he uses: "I am not going to have a Labour government if it means deals or coalitions with the Scottish National Party." Note the exact words: "deals or coalitions". He is not ruling out discussions, consultations, or taking into consideration SNP views when framing legislation.
And there is a clear reason for Labour adopting this stance: to try to convince Labour voters in Scotland that the only way they can be sure of getting Mr Miliband into Number 10 is by voting for his party, not Nicola Sturgeon's lot.
Last night's Question Time audience, together with many commentators, still seem to think he's being disingenuous or worse. Of course, he'll have to do a deal, they say, otherwise he won't be prime minister. Some of his critics on the left think he is recklessly throwing away his best chance of forming a government.
In my view, they're wrong, for the reasons I've set out. By this time next week, we'll know more. Or not, as the case may be.