Sceptics of my longstanding, confident prediction that Hillary Clinton will beat Donald Trump in November often ask the same question – “He’s got this far despite you all writing him off, so why can’t Trump prove you all wrong again?”.

The question is unarguably valid. Like so many others, I simply couldn’t envisage Trump lasting the distance during the primaries. It was one thing to pose as an outsider, in tune with the Republican base. Quite another to put some meaning to that vague brand, build coherent policies and fight a disciplined, organised campaign.

As it turned out, Trump didn’t need any of the latter substance. By dominating the headlines, Trump made the old political rules redundant, turning the primaries into referenda on him. Even though a large percentage of Republicans were never convinced and less than half supported him, just holding a net favourable position against a crowded field was enough to win most primaries.

By crowding out his rivals and undermining them with smears, Trump prevented them from getting an alternative message across. Ted Cruz ran a textbook campaign, only to be blown away by the cult of celebrity.

Lest we forget, the Cruz plan was to befriend Trump, then sweep up his voters when the celebrity candidate imploded. Yet all they heard was ‘Lyin Ted’, ‘CalculaTed’, the ‘anchor baby’ whose wife worked for Goldman Sachs and whose dad was involved in the JFK assassination.

Subsequently the true outsider, whose policy agenda should be perfectly in tune with angry, revolutionary-minded Trump supporters was booed throughout his convention speech while Heidi was ushered out by security. Substance never came into it. If Trump can destroy Cruz like that – not to mention ‘Little Marco’ and ‘Low-energy Jeb’ – why not ‘Crooked Hillary’?

Here’s why. A General Election is nothing like the primaries. The voters are different, with different values and concerns. The media ask different, harder questions, for longer. The opposition hold absolutely nothing back in attacks. We are seeing signs already that Trump could be utterly destroyed in such an arena.

Remember that the Republican race was unique, and it is unlikely that Trump could have won in any other scenario. 17 candidates meant for the first six months, the only voice gaining traction was Trump.

He was underestimated by his rivals. When Cruz and Rubio should have been attacking Trump, they went after each other – wrongly identifying their principal threat. By the time they finally woke up to Trump’s staying power – just before Super Tuesday – it was too late.

Moreover, the literally endless stream of accusations and potential scandals – from his personal life, past business dealings and bankruptcies, employment of illegal workers, Trump University, failure to publish tax returns, inability to demonstrate claimed charitable donations – all came too thick and fast.

In the infamous Miami debate, Rubio threw one bomb after another, while Cruz effectively scrutinised Trump’s total lack of policy coherence. In response, Trump shouted back as many insults as he received. Only the most dedicated of GOP watcher could possibly have kept up with it all, while the average swing voter in Philadelphia probably turned off in disgust (if they were watching at all).

Nevertheless as I argued at the time, those belated attacks did hurt Trump. He underperformed on Super Tuesday expectations and more or less stalled throughout March. A brokered convention became plausible and #NeverTrump was formed.

Trump, however, shrewdly swerved any more debates and the scrutiny slowed until Wisconsin in early April. There, Trump was exposed during a series of set-piece interviews and duly thrashed in the primary. His response to defeat – attacking Heidi Cruz’s appearance on Twitter – should have been the final warning signal about what his campaign would look like.

However, it was too late to reverse the primary process and the GOP establishment reluctantly endorsed, if not wholly embraced, their presumptive nominee. Perhaps they hoped that Trump was actually playing a smart game during the primaries, and would pivot in order to court his new general election audience.

After three months as the presumptive nominee and a catastrophic fortnight since the convention, it is clear that Trump is incapable of such a pivot. The controversies seem to get worse on a weekly basis – from racist remarks about Judge Curiel to attacking a Gold Star family. Most damagingly, his total lack of foreign policy knowledge is being exposed as questions are asked about Russia, Putin and NATO.

All the other scandals have slipped down the news order, although we can be sure that Democrat attack ads will remind every swing voter in the land ad nauseum. Clinton will vastly outspend Trump in that regard.

On past form, it will push anything negative about Clinton – a flawed, beatable candidate – aside and turn the election into a referendum on Trump. Not a good move when the majority already hold an unfavourable view.

I’m very wary of overstating just how attentive these swing voters have been to date. Normally in elections, the undecideds are less engaged early, tending only to focus on detail when the decision becomes imminent. The signs aren’t good. Trump’s convention speech went down worse than any in history and no neutral observer could deny the last few days have been disastrous.

Far from being on the verge of turning around the polls he describes as ‘phoney’, I suspect Trump is polling near his peak already. The greater the scrutiny, the more serious the campaign becomes, the worse he’ll fare. We are looking at a Clinton landslide.