Of the many things that Budget 2012 will be remembered for, the assault on Canadian seniors must surely rank as Number One.

The importance of the change to Old Age Security (OAS) cannot be overstated.

The Stephen Harper regime has decided — without an election mandate, and without any consultation whatsoever — to fundamentally remake the way in which Canadians approach their retirement years.

It’s breathtakingly brave, or breathtakingly stupid, depending on your political affiliation: Harper’s Conservatives have chosen to deny Canadians access to old age benefits until age 67 — and not age 65, as they have been paying for, and expecting, for a half-century.

It is, fundamentally, a reordering of the social contract between citizens and government. It is an undeniably historic move. And all that remains to be seen is whether Canadian senior citizens, and those approaching retirement years, will let Harper get away with it.

From the start, the way in which the Conservatives have handled the communications around the OAS cut has been outrageously — and uncharacteristically — bad. At the annual Economic Forum in Davos, Switzerland, in January, Harper stunned even partisan Conservatives with an announcement that he was considering a rollback in OAS benefits.

OAS was unsustainable, Harper flatly said: “Our demographics also constitute a threat to the social programs and services that Canadians cherish.” The threatening “demographics” Harper was referring to, of course, are flesh-and-blood human beings. Canadian human beings who, more than any other, have worked hard and played by the rules, and expected they would receive a modest amount of support — support that is far outweighed by the taxes, and the labour, they paid into the Canadian social contract.

Harper’s stunning statement — made all the worse by how he did it, before a group of elites and billionaires — was remarkable for another reason: It amounted to a colossal about-face.

Here, word for word, is what Harper said shortly before he became prime minister in 2006: “My government will fully preserve the Old Age Security (OAS), the Guaranteed Income Supplement and the Canada Pension Plan, and all projected future increases to these programs. And we will build on those commitments.”

Well, that was a lie. As of Thursday, the Conservative party has broken its solemn promise to “fully preserve” OAS, and it certainly isn’t planning on “building” on that commitment. Instead, Harper’s wrecking crew has taken a sledgehammer to the entitlement that Canadian seniors want and deserve.

Now, some might argue — and will — that governments and political parties break promises all the time, and nothing ever happens. Lord knows that, in the case of Harper’s Cons, that is true. They seem to get away with political murder on a regular basis.

But this policy mugging of senior citizens is qualitatively different, and for one principal reason: It is politically crazy.

For decades — and particularly since Harper became Conservative leader — Canadian senior citizens have aligned themselves, devotedly, with conservative political choices. Consistently, seniors vote more, and when they vote, they vote Conservative. There is no demographic, to use Harper’s antiseptic word, that has been more loyal to Harper’s political party.

Without them, Harper would have never won government and he certainly would have never won a majority government. Seniors are the very backbone of the Conservative Party. Poll after poll shows that, going back many years.

That is why the gutting of OAS isn’t just a shock, or a whiplash-inducing reversal of longstanding Conservative policy. It is worse than that: It is an attack on the very people who got Harper where he is.

There will be one senior citizen running a political party in the next election campaign. The Liberals’ Bob Rae needs to grab hold of this issue and never let go.

He has been handed a major and unexpected gift. It will be interesting to see if he takes advantage.