Campaigns only last for a few weeks, but politicians can be defined by what they say during those campaigns. Ed Miliband will never live down the #EdStone, Zac Goldsmith will always be stained by his murkier attacks on Sadiq Khan – and I suspect David Cameron will never manage to shake off the threat he made to pensioners today. At times, it can seem as if he’s got confused and has set out to attack his own reputation, rather than that of his opponents.

Those who watched George Osborne grilled by Andrew Neil last week will know the issue: the Remain campaign has decided to pretend that pensioners will be worse off after Brexit. They issued a dishonest poster (above), which overlooks the fact that Cameron himself has promised to protect the state pension with a “triple lock”: it will rise by inflation, earnings or 2.5pc, whichever is the higher. The phrase "triple lock" is designed so David Cameron can say to pensioners: be in no doubt, your pension is safe under the Tories no matter what. You have my word.

It seems that Cameron is regretting this pledge now: those ungrateful pensioners seem to be the most likely group to vote for Brexit. And, worse still, the most likely to actually vote! So it’s time to get a little tough with the oldies. Put a bit of stick about.

Today, Cameron has said that he’d renege on this “triple lock” promise if voters defy him over Brexit. So much for his cast-iron pledge. Here's his rationale:-

“ “If we had a big black hole, we could struggle to justify this special protection any longer. In fact, even if we could justify it morally, it wouldn’t actually be affordable. Not when pensions represent a huge portion of public spending — over £90bn this year — and when inflation is forecast to hit 4pc if we leave Europe. So here is the reality: if we leave, the pensioner benefits would be under threat.”

He has had a "big black hole" since taking over as Prime Minister: it's called a deficit. He promised this "special protection" knowing he'd face this black hole: pensioners were told their pensions were black hole-proof. Now, he is tearing up this pledge.

He's right to say the triple lock won't be “affordable” - it's not affordable now, and that's partly why IDS resigned. But Cameron still goes ahead with it due to his long-held belief that pensioners can be bought. He bribed them ahead of the last general election, and is trying to threaten them financially now. Hence the disgraceful empty purse image (above).

And if he's right - and rates do end up soaring - then surely pensioners would be getting a better return on their savings? It seems that the "triple lock" was not Brexit-proof.

Those of us who consider ourselves supporters of Cameron are appalled by all of this: can't he see his own reputation is hurt when he sinks this low? Yes, Andrew Cooper may be running the Stronger In campaign as a rerun of his disastrous Project Fear in Scotland. But does the PM have to go along with Cooper's trademark nastiness?

In my view, Cameron shouldn't have made the triple-lock pension promise in the first place: it's unaffordable and imposes generational unfairness. But in threatening to abandon his own manifesto pledge if voters go the wrong way on Brexit, the Prime Minister makes a new point that won't be forgotten after the referendum: there's no such thing as a cast-iron Cameron pledge.