James Bowler, the most senior civil servant in the Treasury, may have just hammered the final nail into a misfiring Tory general election campaign.

With Labour more than 20 points ahead in most polls and the Conservatives heading for fewer than 100 seats, there was a sense of desperation for Rishi Sunak ahead of Tuesday’s televised debate. It was a do-or-die situation.

Well, he did and now the party may die.

Mr Sunak’s team alighted on the threat of a £2,000 tax rise for every household as one of his two main attack lines. And, on the night, it worked.

It worked because it took Sir Keir Starmer 26 minutes even to attempt to deny the accusation. That lack of fleet-footedness meant that the headlines this morning in many cases were about Labour’s £2,000 tax rise.

Starmer and Sunak went head-to-head on ITV in the first debate of the campaign (ITV)

But this was less a killer blow and much more a pyrrhic victory for the prime minister, who had claimed on air to the nation that the figures had been verified by the Treasury.

This morning’s revelation of a letter from the most senior civil servant in the Treasury stating clearly that the £2,000 figure cannot be attributed to his department is extremely damaging, if not a final fatal moment for the Tories.

Already seeing their vote split by Nigel Farage and Reform UK, the Conservatives now have a huge credibility deficit worse than the financial one they wracked up in government.

In the end, if a politician proves to be untrustworthy or dishonest then the voters tend to find them out.

Even if friendly newspapers still push Mr Sunak’s line, it will not close a 20-point poll gap and may make things even worse.

The question is what to do now.

One controversial figure that was repeated – a lot – was the widely debunked assertion that Brexit would return £350m a week to the NHS.

Despite attacks on the credibility of that figure, it was repeated over and over, and even emblazoned on a bus. And many within the Vote Leave campaign feel the tactic worked.

The debate was a key moment for Rishi Sunak (Jonathan Hordle/ITV) (PA Media)

Will Rishi do the same? Repeat it, he has.

Within hours the prime minister had reposted an attack advert centred on the £2,000 claim.

But leading pollsters have warned Mr Sunak he risks voters doubting his honesty and recommended he not repeat it.

Luke Tryl, from More in Common, said: “In general this kind of thing tends to muddy the waters rather than leave people any the wiser. So, I suspect some people will still just hear £2,000 tax rises, but for others it might make them start to question Sunak’s honesty. Overall given Starmer didn’t rebut immediately I think it probably does advantage the Tories in the short term, but given the letter they (the Conservatives) would be wise not to repeat it.”

Polling expert and Tory peer Lord Hayward said: “There is a very big risk for him in any claims, if they are seriously undermined… that the audience will doubt what is being said.”

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