Spreadsheet Phil is having a Nick Clegg moment

Chancellor of the Exchequer Philip Hammond making his Budget statement to MPs in the House of Common

Chancellor of the Exchequer Philip Hammond making his Budget statement to MPs in the House of Commons. PA Wire - Credit: PA

Former Liberal Democrat leader Nick Clegg will have little sympathy with the Conservatives' manifesto woes.

The decapitation of his party, which paved a sole path to Number 10 for his former coalition partners, was likely aided by the backlash over his broken tuition fees promise.

A YouTube apology, widely satirised before the last election, failed to quell voter anger and save the seats of many of his MPs.

Now his old cabinet colleague is feeling the heat.

Hours after his first Budget Philip Hammond has faced growing pressure to back down on a £2bn National Insurance hike for the self-employed.

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And the change, which will cost 2.5m self-employed people an average £240 a year, was savaged by the normally Tory-supporting press.

But most damaging for the chancellor - and by association the prime minister - are the headlines screaming about a broken promise.

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Embarrassing footage and social media posts of current senior ministers promising not to raise national insurance before the last election have been dug out.

In fact, there are arguments in favour of the policy. The Resolution Foundation think tank - which campaigns for low-income workers - hailed the NICs rise as 'welcome and progressive', while the influential Institute for Fiscal Studies said it would go only a 'small fraction' of the way to redressing an imbalance in the tax system in favour of the self-employed.

But IFS director Paul Johnson rightly went on to point out how 'foolish' it was of David Cameron to make his election promise not to raise them.

Election manifestos are not a formal contract but, as the Liberal Democrats found out to their peril, they are a moral contract. It is - and should be - politically damaging to break them without a very good explanation.

In a round of post-Budget broadcast interviews, the Chancellor insisted that the decision - along with a cut in tax-free dividend allowances, which will also hit the self-employed - was 'fair and appropriate'.

Moments after the chancellor had taken his seat after delivering his Budget, aides were insisting that despite an explicit manifesto pledge - reiterated time and again by David Cameron and other MPs - the government was perfectly within its rights to hike National Insurance Contributions because it was in the small print of a bill which had been passed not before, but after the election. It was of course unfortunate that in a pre-budget trail Mr Hammond promised to clamp down on confusing small print for consumers.

But there was still no Nick Clegg-style contrition from Mr Hammond.

With Labour in disarray, he may well decide to muddle on through - judging there is no opposition snapping at his heels.

Jeremy Corbyn neglected to ask the chancellor about the broken manifesto promise in his post-Budget reply.

It is a tough task for the opposition leader to respond to the Budget which he has not had advance sight of.

But mitigation aside, there were plenty of rumblings that the announcement might be coming in the days leading up to the Budget.

And in the social media age, the discontent and outcry at the clear manifesto breach was already in evidence well before Mr Corbyn had his turn at the despatch box.

Regardless of the pros and cons of the policy, a manifesto broken promise is a compelling political weapon.

It may seem that manifestos don't matter - but they have a long tradition of being a moral contract.

The Salisbury Convention - which dictates that the House of Lords can use their in-built majority to amend legislation in a manifesto, but not defeat it - demonstrates how seriously they are taken.

Traditionally the documents are closely studied by government department and civil servants regard them with authority.

Manifestos are designed with the clear intention of allowing voters to know what politicians intend to do in government before they cast their ballots.

The Treasury may well have thought it could weather the storm and people might not notice the broken promise.

But it is finding to its peril that is not the case. And in the absence of Mr Corbyn, there is potent opposition coming from the chancellor's own side.

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