When opinion polls first suggested that Jeremy Corbyn had a good chance of becoming the next Labour leader, the response from the right was glee, laced with incredulity that Labour could behave so destructively. A brief period of revisionism followed, with warnings about counting chickens pre-hatch and the dangers of one-party government.

Now, the circle has closed, and the mood on the right is mostly to sit back and enjoy the show. The forecast is that, if Corbyn wins – and it is worth stressing that it is still only “if” – Labour will be out of power for a generation and might not even survive as a major party. And if he doesn’t, Labour will still be so damaged that it will be toothless as an opposition for years to come.

Such is the confidence in government circles that David Cameron and George Osborne are said to be planning a short, sharp autumn offensive under which anti-strike legislation and a host of other measures will be passed nem. con. The calculation appears to be that Corbyn will make no mark, and that any other post-contest leader will be too preoccupied with rebuilding the party to concentrate on the task in hand.

That is the theory. But there are reasons why such a rosy prospect, as it appears from the Government benches, could be quite wrong. Take Parliament first. It is blithely assumed that Corbyn’s somewhat fuzzy, sometimes hesitant air will translate into a poor performance at Prime Minister’s Questions. That need not be so.

Corbyn has one asset that even Tony Blair at the height of his oratorical power did not possess: an ideologically coherent view of the world. His position, barely changed in its fundamentals over decades, is entirely of a piece. You may agree with him or not, but you cannot say you do not know where he stands, or how one view fits with another. The SNP leader, Nicola Sturgeon, impressed during those pre-election debates not just because she spoke with conviction, but because her positions formed a coherent whole. This is why some erstwhile Labour voters were envious.

As Ed Miliband found to his cost, being a moderate exposes contradictions. Corbyn’s rivals for the Labour leadership illustrate this liability of centrism even more graphically. A “foolish consistency may be the hobgoblin of little minds”, but ideological coherence has intellectual appeal. Nor is Cameron immune to accusations of inconsistency in presenting himself as a moderate on the right. The only reason he has mostly not had to confront them so far is that in his first term as Prime Minister, he could blame the Liberal Democrats and then that Ukip made such a mess of its election campaign. If Cameron has to face an authentic leftist at the despatch box, he could find it harder than he or his MPs might think.

As leader, Corbyn could struggle to cobble together a front bench or keep his MPs in line, but presenting his case to the country could be another matter. To argue that a Corbynite Labour Party is unelectable constitutes old thinking along two trajectories. First, leadership. Corbyn himself may indeed not be “electable” as Prime Minister – but he might well recognise that and nominate someone else. It is convention that the party leader is prime minister in waiting, but the functions could be split. Second, what of the view that British voters will never, ever give a left-leaning Labour Party a majority? Surely the future has to be Blairite? But it is now 2015; the Scots booted out most of their Labour MPs in favour of an SNP considerably further to the left, the UK has experienced the banking crisis, the growth of unstable employment and big demographic change. The most elementary of fallacies is to assume that the future will replicate the past.

Which is also why any jubilation on the part of the Conservatives is as ill-advised as it is premature. If – and it is worth repeating that “if” again – Corbyn does become the new face of Labour, it should not be assumed that he will fail. There is a constituency for the old-fashioned left in the country at large, a constituency that has been disguised by the traditional broadness of Labour’s electoral church, by the first-past-the-post electoral system, and additionally now by the Blairites’ loud noises about electability. And if a Corbyn-led opposition were to start striking some blows, there could be knock-on effects for the Conservatives. One would lie in the appeal of Corbyn-ite coherence. Alongside a believer, Cameron could sound perilously wishy-washy, reinforcing the long- standing criticism that he is a lightweight without ideology.

Against this, it can be argued that managerial competence is at least as important for modern government as ideology, and that Cameron’s approach exemplifies this.

But a more ideological opposition could re-cast ideological blandness as a major shortcoming. Cameron might then find it much harder to keep the peace within his party, as he has done with reasonable success on a host of issues, from Europe, through benefit cuts and federalism, to – yes – asylum and immigration.

The Conservatives’ slim parliamentary majority may be the best insurance against these old divisions opening up to destructive effect. But underestimating the extra-parliamentary appeal of Corbynism, taking it for granted that the next Labour opposition will be weak, and possible pressure for ideological coherence on the left to be matched by the same on the right all present dangers to David Cameron. He and his government should be a lot more afraid of a Corbyn victory than their current cheery demeanour suggests.