Both Labour and Tories are making promises but how do they intend to pay for them? - Andrew Vine

In particular, both have spent the first weeks of a so-far underwhelming election campaign demonstrating an aversion to telling us how any of their big ideas are going to be paid for.

So tonight, we can expect two party leaders rehearsed to the max by their PR people, armed with carefully-scripted barbs and soundbites, but as shifty as door-to-door salesmen about coming clean on the cost of anything.

Instead, we’ll get lofty visions and emotive words like change, stability, and prosperity.

We’ll probably get lump-in-the-throat stuff about how much they love their country, and misty-eyed recollections of humble beginnings, one as the son of a toolmaker, the other as that of a pharmacist, and how those upbringings shaped their values.

All of which is fair enough as far as it goes, but it tells us next to nothing about how each would fix the NHS, ensure the old and vulnerable get the care they need or prevent councils going bust.

But then these debates often generate more heat than light. Once in a rare while, they can be illuminating, such as the rancorous exchanges between Mr Sunak and Liz Truss over who would succeed Boris Johnson as Prime Minister.

On that occasion, it was obvious to any impartial observer that Mr Sunak was the voice of reason and Ms Truss a loose cannon with some very strange ideas on the economy. The Conservative Party membership didn’t agree and sent her to Downing Street, a decision largely responsible for Mr Sunak’s current uphill struggle to avoid eviction from it.

The long shadow Ms Truss cast over her party will undoubtedly be raised tonight by Sir Keir, but point-scoring about the past can’t disguise evasiveness about the future.

What we’ve mostly heard from both leaders is aspiration rather than policy. Their ambitions for better health services, education and social care are laudable but vague.

The election so far, and tonight’s debate as well, is about not frightening anybody.

And so there is soothing reassurance from both – no new taxes, no rise in VAT, steadily increasing pensions. And that equates to no answers to the questions most of us have about the size of the bill and paying for it.

Currently, there is no detail on that beyond both parties saying they will crack down on tax evasion, one of the oldest political cliches in the book and the least believable.

Every party at every election I can remember has promised to go after those who don’t pay their fair share, and yet it has never produced a magical pot of money to cure the ills of the NHS, give schools the funds they need or enable councils to deliver good local services.

Maybe there is something like £6bn in unpaid taxes floating about as ministers have suggested, but for Mr Sunak in particular, the question arises of why his party hasn’t hunted it down over the last decade as the squeeze on public finances grew ever tighter.

And even if that sort of sum could be recovered, it’s peanuts in national terms. To place it in context, a House of Lords report a few weeks ago put the bill for outstanding maintenance work on NHS buildings at nearly £12bn.

Touting unpaid taxes as the solution to the nation’s financial headaches is disingenuous, no more than an attempt to look tough and pander to people’s sense of grievance about dodgers who get away without paying what they owe.

Whether or not they admit it tonight, at some point Mr Sunak and Sir Keir are going to have to spell out the cost of what they are outlining.

You don’t need to be an economist to know that in the absence of a sudden and sustained spurt in growth, the basic means for paying for whatever the next Prime Minister sets out to do are higher taxes, borrowing, or cuts to existing budgets, all of which will mean pain for the public.

Labour have committed to sticking to existing financial targets if elected, which already include projections for cuts to Government spending.

Opposed though they are, both men on stage tonight are to an extent bound to the same fate because of their ultra-cautious determination not to alarm the electorate.