As Rishi Sunak was winding up his manifesto launch at metaphor-heavy Silverstone race track, the scale of the prime minister’s task in the remainder of the election campaign was becoming clear.

According to the exclusive Sky News-YouGov poll, Sunak needs to go through the gears at once or he’s in danger of dropping to the bottom step of the podium.

He was speaking hours before it emerged voting intention for the Conservatives had dropped to the joint lowest in this parliament – 18% – now putting Sunak’s party just one point ahead of Nigel Farage’s Reform UK on 17%, tantalisingly close to a crossover.

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A full third of 2019 Tory voters – the cohort that endorsed Boris Johnson last time – now say they will switch to Reform UK, while the proportion who think Sunak will be a good prime minister is down two points – to 22% – in the last fortnight. That last figure is possibly a casualty of the PM’s decision to leave D-Day commemorations early – and could conceivably have been worse.

The notable drop in Labour’s vote – three points to 38% – will do little to cheer a Tory party in the doldrums, consumed with their own existential angst. This is because the switch seems to match the Lib Dems jumping up four point to 15%. Much of the YouGov fieldwork was done when the Lib Dem manifesto was receiving peak coverage.

The question is whether the Tory manifesto launch could possibly have provided anything new with which to turn things around, from a position as dire as any Conservative can remember in living memory.

Sunak has thrown everything at this manifesto: it’s 72 pages long, with nearly £20bn worth of tax and spending announcements.

There are pledges designed to appease and appeal to just about every demographic, from 2p off national insurance for working families, to accelerated national insurance abolition for the self-employed, to tax cuts for pensions, to help for first-time buyers and tax breaks for wealthier parents. This is to be paid for, Sunak said, in large part by yet more promises to pare back welfare, squeeze the public sector and more anti-avoidance measures.

It is a “kitchen sink” manifesto for the Tories. But it is not the first “kitchen sink” manifesto in recent memory.

Sir Keir Starmer boldly compared Sunak’s offering with that of Jeremy Corbyn – stuffed with policies that seem, and poll as, popular but are not sustainably affordable as an overall package.

The Labour leader was, of course, displaying the chutzpah of a man 20 points ahead in the polls by casually disowning a manifesto he himself stood on five years ago.

Nevertheless, his political purpose by making this point is two-fold: firstly, he is attempting to needle away further at the Conservatives’ claim of economic credibility, while also reminding people that manifestos stacked with popular policies do not automatically win elections.

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But despite having individually popular ideas, the prime minister was unable to deliver perhaps the biggest thing Tory MPs might have wanted – a promise to reduce the overall tax burden in the next parliament.

It is the tax burden that hangs around the neck of a party proud of its low tax heritage, at an event at which Sunak had the audacity to invoke Nigel Lawson, the 1980s tax-cutting chancellor.

Sunak cannot bring it down. Yet he is unwilling to be completely automatically transparent over this point.

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Examine carefully this painful exchange in the questions from the media afterwards, when Sunak’s sleight of hand was noticeable.

He was asked by the Daily Mail: “Can you today guarantee that if you get in, overall taxes will be lower by the time you finish?”

To this, Sunak replied: “Because of the measures that are announced in the manifesto and you can see that document afterwards, the tax burden will be about one percentage point lower in every single year compared to the forecast that you saw at the spring budget a few months ago that Jeremy (Hunt, the chancellor) outlined.”

This answer is deliberately elliptical, because the truth is hard: more people are dragged into higher tax bands because of frozen thresholds, designed to pay back some of the debt incurred in COVID.

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As my colleague Ed Conway, Sky’s economics editor, says, even after the tax cuts in this manifesto, everyone will still be paying higher taxes in 2028-29 than we are today.

So the answer to the Daily Mail is yes – the tax burden will be higher, albeit not as high as previously planned.

Sunak’s answer, while true, made it sound like the picture is better than it is when it comes to tax – and it’s a complication for a Tory leader trying to make tax the key dividing line with his Labour opposition in this election.

As Beth Rigby pointed out, a recent poll shows that only one in six voters believe Sunak won’t raise their taxes, or raise major taxes, compared with one in four for Labour – and she asked him whether this means he has “blown it”.

“I’m not afraid to do things that are difficult,” he pleaded in response. It’s not clear many on his own side believe this argument will wash with the public at this late stage in the political cycle.

Rishi Sunak and Sir Keir Starmer will face questions from Beth Rigby and members of the public during Sky News’ special leaders’ event on Wednesday.

The programme airs live from Grimsby from 7pm on Sky News – Freeview channel 233, Sky 501, Virgin 603, BT 313 – and streaming on the Sky News website, app and across social channels. It is also available to watch on Sky Showcase.