As David Cameron prepared to launch his pitch that the Tories were now “the party of the NHS” with publication of the first chapter of their draft manifesto, Alistair Darling took to the podium at Labour HQ to rubbish the Conservatives’ spending plans.

The Chancellor claimed more than £45bn of expenditure had been set out by the Tory Opposition but only £11bn in savings and tax rises had been set out to pay for it; meaning a “credibility gap” of £34bn.

This, argued Mr Darling, meant the Conservatives were planning more tax rises or deeper spending cuts.

“You can’t fight an election on a nod and a wink, sometimes claiming you are committed to promises and, when challenged, claiming you are not. It’s for them to say when and how they’ll be paid for or come clean and withdraw them,” he said.

Tory HQ swiftly dismissed Labour’s 150-page dossier as “complete junk from start to finish”.

George Osborne accused Labour of peddling “lies”. The shadow chancellor claimed the dossier “includes commitments we have never made, wild exaggerations of our costed policies and in some cases, admissions that some changes would actually be cheaper than we have budgeted for”.

Meantime, Mr Cameron sought to shift the focus to the NHS, promising to tackle the gap in healthcare in England between rich and poor by diverting cash to the worst-off areas through a new health ­premium. “We are the only party committed to protecting NHS spending. I’ll cut the deficit, not the NHS,” insisted the Tory leader, launching a poster campaign.

However, Mr Cameron suffered from the first campaign gaffe. In an interview, he appeared to row back from his commitment to recognise marriage in the tax system should he win power by saying this was something he “definitely hoped” he could do but would not make “false promises” about.

Later, he issued a statement, reaffirming that recognition of marriage in the tax system was something he felt “very strongly about and something we will definitely do in the next parliament”.

In December, The Herald ­reported how senior party sources made clear that any tax breaks for married couples would come at the end rather than the beginning of the next parliament because of the need to scale back the giant budget deficit.

Last night, Labour seized on the Cameron wobble with Liam Byrne, Chief Secretary to the Treasury, claiming in one afternoon the Tory leader had had three positions on the ­ policy, adding: “The Tories are clearly panicked and in disarray over promises they made but can’t afford on tax cuts and spending commitments.”

Elsewhere, the Liberal Democrats criticised the Conservative proposal to raise the inheritance tax threshold, exempting estates worth up to £1m, saying it was “unbelievable” when most people were feeling the squeeze while the SNP accused the two main UK parties of waging a “phoney war”, that excluded Scotland.

Today, the Scottish Conservatives’ Annabel Goldie and David Mundell will begin touring the party’s 11 target seats in Scotland while party workers will be delivering copies of the party’s newspaper “Changing Times” at railway stations across Scotland.